Putin's Preventive War: The 2022 Invasion of Ukraine

Russian President Vladimir Putin's decision to invade Ukraine in 2022 is consistent with the logic of preventive war.¹ Simply put, one reason states often initiate wars is because they fear the consequences of a shifting balance of military power.² In the logic of preventive war, the declining state worries that an existing competitor may initiate war later under more favorable circumstances, or that a rising state may use its newfound muscle to coerce the declining state. Notable preventive wars since World War II include when China intervened in Korea in 1950 to prevent the United States from unifying the Koreas and settling its own forces on the Chinese border; Israel's attack on Egypt in 1956 to forestall its absorption of a huge new supply of tanks and fighter aircraft from the Eastern Bloc; and the U.S. invasion of Iraq in 2003 to forestall Iraq from possibly acquiring weapons of mass destruction.

The tendency to consider preventive war is exacerbated if the declining state simultaneously sees itself as having a special, and fleeting, window of opportunity to prevent the shift. Putin likely believed that Russia faced such a moment. Ukraine's membership in the North Atlantic Treaty Organization (NATO) would irretrievably shift the balance of power against Russia. During the Joe Biden administration, the United States and NATO intensified training, arming, and advising Ukraine, activities that likely convinced Putin that he did not have much time to forestall NATO membership through military action at reasonable cost. Offering this very logic, Avril Haines, the U.S. director of National Intelligence, explained that Putin attacked because “military action would be the best remaining option to prevent greater Ukrainian integration with the West, which he believed to be a significant threat to Russia's national security. Furthermore, given the trend lines, it would only get more difficult to affect a military option over time.”³ Even Jens Stoltenberg, the secretary general of NATO, understood Russia's preventive motive: “So he [Putin] went to war to prevent NATO, more NATO, close to his borders.”⁴

Historians will debate whether it was Putin's strategic calculus or his commitment to a nationalist or imperialist ideology (or some combination) that best explains the 2022 war. In the months since the war began, discussion of a strategic motivation has atrophied. This is unfortunate because if the Russians did have strategic motives, even in part, then understanding these motives could contribute to the diplomacy of an ultimate war settlement.⁵ Additionally, looking at the invasion through the lens of preventive war theory may provide some policy lessons for other dangerous conflicts of interest.

This article takes the form of a “case explaining” case study.⁶ Theory is mobilized to better understand murky events. In contrast to most political science case studies, which treat the credibility of a theory as the matter at issue and the case as valid evidence that supports or undercuts the theory, here the theory is treated as valid, and the “facts” of the case are at least partly at issue. Put another way, there are many facts, but there is an uneven and incomplete supply of them, and analysts use the theory to make sense of those facts that do exist. I also use the theory as a coarsely ground lens to spy out some facts that may be hidden in the weeds.

An extant general theory is an excellent guide to structure disparate evidence. Theories make predictions—it is possible to organize the events of the case to see if they conform to these predictions. Importantly, if the going-in theory is itself a strong one, and many of its predictions are confirmed, then that explanation for the case becomes more credible, in part because the initial theory is itself strong. If the theory is deductively sound, and more importantly has explained historically similar events, scholars are entitled to give the explanation extra weight in understanding the case at hand, if they can show that the case looks like this class of events.

Thus, I first review the theory of preventive war and briefly consider what scholars believe about its prevalence historically. This theory is itself deduced from realist international relations theory, which endures as the strongest explanation of patterns in international politics. Then, I assess whether the major predictions from the theory are present in the case.

The most general prediction of the theory is that the growing prospect of Ukrainian membership in NATO would seem threatening to Russia in the eyes of the Russian foreign policy establishment, not merely in the eyes of Putin. The prediction is credible because most great powers have historically viewed other great powers on their borders as a threat, and NATO membership for Ukraine would put the United States squarely on Russia's doorstep. In the post–Cold War period, many Western experts on Russia and national security predicted for years that Russia would see NATO enlargement as a threat.

Second, the theory predicts that declining states should note their changing status and attempt to reverse it if they can. Russian leaders told the West that they would view NATO on their doorstep as a threat. Russian diplomacy and published strategy documents expressed security concerns. They tried to warn NATO off.

Third, the theory predicts that the declining state's fears should have some basis in fact. I will show that the on-the-ground changes in Russia's military situation that would attend Ukrainian membership in NATO would prove problematical for Russian security. Moreover, the history of U.S. and NATO military deployments during and after the Cold War suggested strongly that capabilities dangerous to Russia would ultimately move forward into Ukraine alongside NATO membership. Indeed, NATO forces were already regularly turning up in Ukraine. Russian spokespersons and documents warned repeatedly that they observed these developments, feared them, and explained why they feared them. I review evidence to show that their fears of the specific military implications of NATO's presence in Ukraine were reasonable.

Fourth, the theory suggests that Putin and his advisers ought to have had evidence that NATO membership was a genuine prospect and not simply a talking point. I discuss why Russian observers could have viewed the prospect of NATO membership as an impending development that had been gathering momentum since NATO announced future membership for Ukraine in 2008. Although a steadily deteriorating power position produces preventive motives, growing momentum helps confirm the fears of the declining state. The history of NATO's relations with Ukraine, reviewed below, also suggested to Russia that ultimate membership was likely. Some have argued that Russia had to know that NATO membership was not likely because there was no Membership Action Plan (MAP), the last hoop NATO aspirants once had to jump through. I will show that NATO was forging so many other connections with Ukraine that the absence of a MAP would not have reassured Russia much.

The theory also predicts that the window of opportunity to do anything about the emerging problem is closing. I will discuss the extent to which Russian observers likely viewed Western military cooperation with Ukraine as proceeding at a pace that would make it increasingly difficult for Russia to do anything militarily to prevent Ukraine's membership in NATO. “War now,” appeared to be better than “war later.” NATO and NATO member states’ military relations with Ukraine deepened Ukraine's integration with NATO and improved Ukraine's combat power, and Russia noticed.

One caveat is in order. A preventive war theory cannot explain the exact moment when a war begins, and I do not try to do so. Instead, it indicates whether the state faces a growing incentive to act. These pressures mounted in the years immediately preceding the war, as Geoffrey Roberts demonstrates in his perceptive treatment.⁷ The choice of war is a major decision. Leaders such as Putin must contemplate how they and the state they lead will fare in such a war. One important question leaders must ask is simply whether the military can come up with a plausible plan for victory.8 I do not take up this issue in detail, in part because these internal deliberations are hidden from most of us, though Western intelligence may possess evidence that bears on the question.

The article proceeds as follows. The first section reviews the extant theory of preventive war from which I have inferred the predictions above. The second section addresses ambiguities in extant preventive war theory that blur the difference between efforts to forestall windows of vulnerability and states’ temptations to exploit windows of opportunity. I then review the evidence of the preceding twenty-five years to demonstrate the extent of its conformity with the predictions of the theory. I close with a brief review of competing theories for why Russia invaded Ukraine and some implications for policy.

Many international relations scholars consider preventive war to be a fact of life in international politics.⁹ This group is comprised of political scientists, such as myself, who are realists of one sort or another, but also diplomatic and military historians.¹⁰ “Preventive war, however abhorred in diplomatic language and abhorrent to democratic public opinion, is in fact a natural outgrowth of the balance of power,” Hans Morgenthau instructed us in his classic mid-twentieth-century work, Politics among Nations.¹¹ In a world without governments and police forces, any state can do to any other whatever its power allows. States therefore seek power at minimum to protect themselves, but also to pursue their interests against those who would block their way. Under these conditions, states are sensitive to changes in the balance of power. When some states develop alliances, build bases, seize or negotiate privileged access to strategic real estate, and increase the size and quality of their militaries, others take note. Because each state must look after itself, states often do not wait to see how well others will do as they endeavor to increase their power.

Realist theory predicts this sort of behavior, and history suggests it is common.¹² States act preventively to forestall reversals of power relationships.¹³ Scholars of preventive war routinely cite Thucydides's history of the Peloponnesian War to illustrate the logic: As quoted by Alfred Vagts, “Men do not only defend themselves against a superior when he has attacked them, but also strike the first blow, to prevent his attacking them.”¹⁴ It is important to stress again that these theories are not hortatory: They do not advise statesmen to have preventive wars. These are predictive and explanatory theories. Preventive wars happen because states are generally insecure; given that they live in an anarchical world, they are on the lookout for looming threats to their security.

Many scholars agree that the preventive motive for war is strong. They come to this view either through studying many wars or focusing on particular cases that demonstrate the power of the preventive motive relative to others.¹⁵ In any given case, scholars may disagree on both how many wars preventive motives help explain and the extent to which preventive motives explain why a state initiates war. From 1650 to 1990, a great power initiated a conflict for preventive reasons in a third of the wars (twenty out of about sixty involving at least one great power).¹⁶ Included in this list are all the world wars: the Seven Years War, the French revolutionary and Napoleonic Wars, World War I, and World War II. Stephen Van Evera, in a careful survey, finds nineteen preventive wars, from the Peloponnesian War through the 1991 Persian Gulf War, to which I would add the 2003 U.S. invasion of Iraq and the 2022 Russian full-scale invasion of Ukraine.¹⁷ Quantitative studies also suggest that there are many preventive wars, though scholars disagree on whether preventive motives are a strong cause of war.¹⁸

Preventive wars are thus theoretically predicted and commonly practiced, and in modern times, legally proscribed, though that has not kept even liberal states from undertaking them. Ironically, practitioners and theorists also generally view preventive war as unwise—prone to go awry. Those advising against it often paraphrase Otto von Bismarck's famous adage that preventive war is like committing suicide for fear of death.¹⁹ (It is noteworthy, however, that even Bismarck found the idea of preventive war attractive earlier in his career.)²⁰

There is, to my knowledge, no comparative treatment of preventive wars that failed. But scattered observations in the literature on preventive war, and deduction from extant theories, provide some insights. First, those who have the capability to launch preventive war are often, by definition, already strong. Thus, they are watched warily by others, who for their own security reasons will be disinclined to give a bold move the benefit of the doubt. In multipolar systems, other great powers are likely to join the victim if they have the wherewithal and the time: Balancing happens.²¹ Second, those undertaking prevention may not understand that others base their decisions to form alliances on not only the power of a potential adversary but also perceived malign intent. Initiating war tends to make a state appear malign, even if the initiator had a pretty good reason. Stephen Walt notes that states balance against threats, and in addition to power, a component of threat assessment is how states perceive malevolent intent.²² Third, military defense often has an inherent advantage, as Carl von Clausewitz argued.²³ Wars seldom go exactly as planned. And failure to achieve a quick victory gives the preceding balancing mechanisms a chance to work and bring assistance to the target's side. Finally, and perhaps ironically, states may understand that preventive war is a bit of a desperate measure, and thus they wait too long. The balance of power is indeed moving against them, and that shift turns out to be already well underway.

States considering preventive war have alternatives. They can dig deeper into the national purse to try to generate more material power by investing in either their militaries or their economies. They can fortify their borders. They can court alliances themselves. And they should always consider that the momentum of their adversary's rise may simply slow or perhaps even stop. The state should compare whether the costs and risks of initiating war are greater than exploiting political and military defensive measures.²⁴ That said, these alternatives are uncertain. Even states that consider all these nostrums, and are aware of the pitfalls, could choose preventive war after calculating the risks and costs. Foreign policy is not an exact science.

Western official sources regularly assert that Russia was unprovoked when it invaded Ukraine. It was not a preemptive war, they say. Preemptive wars—when one party attacks because it has good reason to fear imminent attack by another—are legitimate in international law, but this occasion does not often arise.²⁵ Putin had no reason to fear imminent attack on Russia. In that sense, the war was unprovoked, and by modern standards, entirely illegitimate. Though preventive wars are historically common, they enjoy little support in modern international law.²⁶ The United Nations Charter outlaws preventive war, if only because it outlaws states from initiating almost any war: “All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.”²⁷ Moreover, the charter confers the right of preventive action to the UN Security Council. Thus, if the argument below convinces readers that Putin's motives were preventive, and that he came by these motives through a reasonable strategic appraisal, this does not legitimate his war. It does help explain it.

Followers of both international relations theory debates and foreign policy debates will be familiar with the terms “window of vulnerability” and “window of opportunity.” In the words of Randall Schweller, “According to this broad definition, states wage preventive wars for either offensive or defensive reasons: to take advantage of a closing window of opportunity or to prevent the opening of a window of vulnerability.”²⁸ These terms relate to preventive war, but all windows are not the same, though theorists tend to treat them that way. Though this discussion may seem obscure, it sheds light on the complexity of preventive war in general and Russia's invasion of Ukraine in particular.

Preventive motivations for war arise in the face of looming windows of vulnerability—changes in the balance of power that may increase significantly a state's future exposure to a rising power's military action. Thus, the declining state fears that its future military security and bargaining power could be vastly and possibly irretrievably reduced. The declining state expects to face greater danger of military defeat or coercion. At the same time, the declining state perceives that the present balance of power allows a reasonable chance of success in a preventive attack. Tragically, depending on how dire the future appears, even a throw of the dice may appear reasonable to the state contemplating an attack.

Preventive war becomes even more likely when the declining state perceives its chances of success to be quite good (i.e., it has a window of opportunity). In the words of Van Evera: “Window of opportunity vs window of vulnerability. The former is a fading offensive opportunity, the latter is a growing defensive vulnerability. A single window can be a window of both opportunity and vulnerability, if the declining state expects to fall all the way from dominance to helpless incapacity.”²⁹ Sometimes the balance of power may afford this window—things are good now and will be awful later. Sometimes events may afford it—the rising power's best ally is suddenly bogged down in another war. Or the rising power itself is suddenly bogged down in another war. Or the declining state's allies are unusually willing to cooperate to thwart the rising state.

Windows can also be about securing an important stake in a dispute between two great powers. A vital piece of real estate, in some cases a whole country, might add greatly to another state or coalition's power; it also might add greatly to the power of the state considering preventive attack. If one power sees another state drifting to the other side, or vulnerable to occupation by that side, it may strike to prevent the change in the balance of power and indeed to seek an advantage for itself. The Soviet attack on Finland in 1939 was specifically about getting a geographical buffer to help protect St. Petersburg (then Leningrad) from future attack, not by Finland but by Germany.³⁰ Adolf Hitler attacked Norway in 1940 to secure Germany's imports of Swedish iron ore via Norway's ice-free ports because he feared that the Allies were considering occupying Norway to deny Germany those ports.³¹

The term preventive war should be reserved for when a state perceives a looming and irretrievable shift in the balance of power. I argue that this is how Vladimir Putin and many members of the Russian security establishment saw the prospect of NATO membership for Ukraine. There is, however, what might be thought of as an aggressor's window of opportunity. Because it is often hard even for historians and analysts to penetrate a state's motives, or to predict its future motives, scholars usually view wars of this type as preventive wars. A state may simply be greedy for a conquest because it adds to its wealth or power, and sometimes it has a window of opportunity to take the prize now rather than later or not at all. Japan's attack on Wilhelmine Germany's Asian possessions in 1914 is one such example. A window may be fleeting, and the imperialist is desperate to exploit the moment. Even though it may be difficult to characterize preventive motives, conceptually it would be helpful if historians, theorists, and social scientists distinguished between when states launch preventive wars largely for security or when they do so for aggrandizement.

It would have been surprising in historical and theoretical terms had the Russian foreign policy establishment treated the eastward movement of NATO toward its borders with anything other than concern, as this has been the normal behavior of great powers. Regardless of regime type, they all want buffer zones. Stephen Van Evera observes, “History shows that states almost universally resist the close approach of unfriendly powers and alliances toward their borders.”³² The United States, in particular, has been living this generalization since the Monroe Doctrine of 1823, when the United States announced that it would not tolerate European great powers establishing a presence in the Western hemisphere.³³ Though the doctrine itself has become something of a political football that the last two Democratic Party administrations have publicly disavowed, the principle that the United States will forcefully oppose outside powers’ involvement in this hemisphere remains strong.³⁴ Van Evera notes multiple instances when the United States intervened militarily, threatened the use of military force, or intervened covertly to forestall other states from extending their influence in the Western hemisphere.³⁵

The impulse that states feel to keep other great powers at arm's length accounts for “spheres of influence” and “buffer zones” in international politics.³⁶ Many, though not all, spheres of influence arise from powerful states’ propensity to dominate their immediate neighbors, often for security reasons. Michael Reisman, an international lawyer, coined the term “critical defense zone” for this phenomenon.³⁷ Even the European Union (EU), in its first published foreign and security policy in 2003, quickly found itself speaking in terms that would be familiar to nineteenth-century statesmen, though cast in the language of twenty-first-century liberal internationalism: “Our task is to promote a ring of well governed countries to the East of the European Union and on the borders of the Mediterranean with whom we can enjoy close and cooperative relations.”³⁸ Similarly, competing states have often sought to establish buffer zones between them to ensure against the escalation risks associated with having military forces in daily contact.³⁹ Though she rejects the legitimacy of this logic, the longtime scholar of Russian foreign policy Angela Stent observes, “[Putin] seeks acceptance of Russia's right to a sphere of influence in the post-Soviet space, which means that Russia's neighbours have limited sovereignty and cannot join Euro-Atlantic structures because these are deemed a security threat to the Russian heartland.”⁴⁰

Highly knowledgeable analysts forecasted that Russia would react negatively to NATO enlargement almost as soon as the project got underway in the 1990s.⁴¹ In a May 10, 1995 meeting with President Bill Clinton at the Kremlin, President Boris Yeltsin opposed NATO enlargement: “To agree to the borders of NATO expanding toward those of Russia—that would constitute a betrayal on my part of the Russian people.”⁴² In a 1997 letter to Clinton, fifty Cold War foreign policy experts, politicians, and practitioners—a cross section of hawks, doves, and Russia experts and generalists—all agreed: “In Russia, NATO expansion, which continues to be opposed across the entire political spectrum, will strengthen the nondemocratic opposition, undercut those who favor reform and cooperation with the West, bring the Russians to question the entire post–Cold War settlement, and galvanize resistance in the Duma to the START II and III treaties.”⁴³ Already in February 1997, George Kennan—diplomat, strategist, historian, and author of the U.S. Cold War strategy of containment—warned starkly against NATO enlargement: “Expanding NATO would be the most fateful error of American policy in the entire post-cold-war era…. Such a decision may be expected to inflame the nationalistic, anti-Western and militaristic tendencies in Russian opinion; to have an adverse effect on the development of Russian democracy; to restore the atmosphere of the cold war to East-West relations, and to impel Russian foreign policy in directions decidedly not to our liking.”⁴⁴

Later that year, Kennan warned in a private letter to the then–Deputy Secretary of State Strobe Talbott that a planned multinational naval exercise, “Sea Breeze,” sponsored by Ukraine, would trespass on “no more sensitive a point in all Russian diplomatic and military history of the modern era than the question of the entry of foreign warships into the narrow waters of the Black Sea.”⁴⁵ Kennan's warning was prescient—Russian officials protested the first exercise and did so periodically when the exercise was repeated twenty times through 2021.⁴⁶ One did not need to know who would be leading Russia in later years or their ideology to imagine that such leaders would likely oppose the project then getting underway and view it as a major security threat.

Russian leaders have opposed both NATO enlargement in general and enlargement to include Ukraine in particular since these prospects were first broached. Western experts on Russia were aware of this neuralgia, warning in 2008 that enlargement to include Ukraine and Georgia would further alarm the Russians. Fiona Hill, then an intelligence officer on Russia, stated in a brief to President George W. Bush “that Mr. Putin would view steps to bring Ukraine and Georgia closer to NATO as a provocative move that would likely provoke pre-emptive Russian military action.”⁴⁷ U.S. Ambassador to Russia William J. Burns shared similar views with his bosses in Washington in an email drafted in February 2008: “Ukrainian entry into NATO is the brightest of all red lines for the Russian elite (not just Putin)…. I have yet to find anyone who views Ukraine in NATO as anything other than a direct challenge to Russian interests.”⁴⁸ He echoed these observations in an official cable, “Nyet Means Nyet, Russia's NATO Enlargement Redlines,” in which he concluded that “Russia's opposition to NATO membership for Ukraine and Georgia is both emotional and based on perceived strategic concerns about the impact on Russia's interests in the region.”⁴⁹

Russian officials warned repeatedly of their concerns about the prospects for Ukraine's membership in NATO. The then–Deputy Prime Minister Dmitry Medvedev explained to the Financial Times, “We are not happy about the situation in Georgia and Ukraine. We consider that it is extremely troublesome for the existing structure of European security…. No state can be pleased about having representatives of a military bloc to which it does not belong coming close to its borders.”⁵⁰ Despite these warnings, NATO promised future membership to Ukraine and Georgia at the 2008 Bucharest Summit: “NATO welcomes Ukraine's and Georgia's Euro-Atlantic aspirations for membership in NATO. We agreed today that these countries will become members of NATO.”⁵¹ Russian Defense Minister Anatoly Serdyukov told Finland's National Defence Journal, “We stand for a serious, constructive dialogue with the alliance…. But if someone hopes we will compliantly watch a military armada being built at our borders, with our national security interests being ignored, then we will have to upset someone.” Serdyukov questioned “the stubborn persistence of some alliance members, in particular the United States, to drag Ukraine and Georgia into NATO, no matter what. We would like to know, for instance, whether you would feel worried if there was a person armed to the teeth standing at the door of your house, who would not say why he came or what he would do next, and only brandished a knife in front of your face.”⁵² Putin's views remained stable—in his 2014 speech defending Russia's actions in Crimea, he noted that “we have already heard declarations from Kiev about Ukraine soon joining NATO. What would this have meant for Crimea and Sevastopol in the future? It would have meant that NATO's navy would be right there in this city of Russia's military glory, and this would create not an illusory but a perfectly real threat to the whole of southern Russia.”⁵³

Putin's concerns about NATO enlargement were echoed in his UN speech the following year.⁵⁴ In 2018, in an address to Russia's own ambassadors assembled in Moscow, he warned against the “aggressive steps” of including Ukraine and Georgia in NATO.⁵⁵ In his July 30, 2021 radio call-in he averred, “I'm worried about something more fundamental—namely that their military forces are making themselves at home on Ukrainian territory.”⁵⁶ In response to the 2008 promise of membership, Putin then declared that “the appearance of a powerful military bloc on our borders would be taken in Russia as a direct threat.”⁵⁷ In 2024, NATO remained adamant that “Ukraine will become a member of the Alliance. NATO supports the right of every country to choose its own security arrangements, including Ukraine. NATO's door remains open. Ukraine, as the country who wishes to join and NATO Allies decide on NATO membership. Russia does not have a veto.”⁵⁸

Russia's opposition to NATO enlargement has been a feature of its official security documents since 2000. Observers of Russian security thinking agree that the 1999 Kosovo War marked an inflection point, after which Russia was far more suspicious of NATO's actions and intentions.⁵⁹ Russia's National Security Concept, published in early 2000, viewed as “fundamental threats,” the prospect of “the strengthening of military-political blocs and alliances, above all NATO's eastward expansion” and “the possible emergence of foreign military bases and major military presences in the immediate proximity of Russian borders.”⁶⁰ The complimentary Russian Federation Military Doctrine did not explicitly mention NATO, but viewed as “main external threats … the creation [buildup] of groups of troops [forces] leading to the violation of the existing balance of forces, close to the Russian Federation's state border and the borders of its allies or on the seas adjoining their territories,” and “the expansion of military blocs and alliances to the detriment of the Russian Federation's military security.”⁶¹ Subsequent versions of the doctrine, in 2010 and 2015, were more explicit, listing the “main external danger” as “the desire to endow the force potential of the North Atlantic Treaty Organization (NATO) with global functions carried out in violation of the norms of international law and to move the military infrastructure of NATO member countries closer to the borders of the Russian Federation, including by expanding the bloc (emphasis added).”⁶²

Russian leaders and commentators echoed this theme in the fall of 2021. At the Foreign Ministry in November, Putin alluded to NATO's nuclear exercise in the Black Sea region, “Steadfast Noon,” complaining that the alliance was “showing a markedly confrontational attitude, persistently, flagrantly bringing its military infrastructure closer to our borders (emphasis added).”⁶³ He noted later “how close NATO infrastructure has moved to our border. This is extremely serious to us.”⁶⁴ At a December Organization for Security and Co-operation in Europe (OSCE) meeting, Russian Foreign Minister Sergey Lavrov warned that “turning our neighboring countries in a springboard for confrontation with Russia is unacceptable, as is the deployment of NATO forces in the immediate vicinity of areas that are of strategic importance to our security.”⁶⁵ Andrey Kortunov, director-general of the Russian International Affairs Council, said that Russia troop movements were “a demonstration of Russian's concern over what Putin has referred to as the development of NATO's military infrastructure on Ukrainian territory.”⁶⁶

The Russian impulse to seek a security buffer on its periphery is not simply the instinct of any great power, nor is it merely a nostalgic effort to restore some semblance of the Romanov or Soviet Empire. NATO membership for Ukraine would materially affect Russia's security in a narrow military sense. Russia would naturally fear that membership in NATO would bring NATO military and intelligence assets forward into Ukraine. This is what happened in former Warsaw Pact allies of the Soviet Union that joined NATO. The Russians would have no reason to expect anything different in the case of Ukrainian or Georgian membership in the alliance. For example, the U.S. ballistic missile defense installation in Poland proceeded despite Russian complaints.⁶⁷ Though the United States assured Russia that the system had little capability against it, a Russian military planner might find the following statement about Aegis Ashore arresting: “Mission: Provides a forward-deployable, mobile, and Aegis Ashore capability to detect and track missiles of all ranges in all phases of flight with the ability to destroy missiles in the midcourse and terminal phases.”⁶⁸

Russia has laid out its worries in its military doctrine. The drafters were particularly concerned about actions that aimed “at hampering the work of Russian systems of state and military rule, or at disrupting the functioning of strategic nuclear forces, missile-attack early-warning, antimissile defense, and space monitoring systems and systems for ensuring their combat stability.”⁶⁹ Russia's fears about the survivability of its nuclear forces and the command and control that allows them to operate are not misplaced. U.S. nuclear doctrine during and after the Cold War called for being able to attack Russian strategic nuclear forces during wartime in order to limit damage to the United States.⁷⁰

Command and control has long been deemed the weak link of strategic nuclear forces.⁷¹ The 2010 Russian Federation Military Doctrine continued in this vein, characterizing as a “main military threat” the “impeding of the operation of systems of state and military command and control of the Russian Federation, the disruption of the functioning of its strategic nuclear forces, missile early warning systems, systems for monitoring outer space.”⁷² The same publication noted that one of the “characteristic features of contemporary military conflicts are the reduction of the time parameters for preparing to conduct military operations.”⁷³ Kortunov argued that “it appears that the real, and not mystical, fears are connected with the possibility that the bloc's [NATO's] military infrastructure will be moved closer to Russia's borders. Experts often say that the flight time for US cruise or ballistic missiles to reach Moscow will shorten significantly in the event of the alliance's ‘military colonization’ of Ukraine.”⁷⁴

It is likely that Russia's lessons from the 1999 Kosovo War prompted these specific fears, at least in part. Though perhaps hyperbolic, Russian analyst Alexei Arbatov observed that the NATO air campaign “left the Russian people with a vivid image of a possible future scenario—with Russia on the receiving end of surgical strikes against industrial, infrastructure, and military targets. These strikes would be especially targeted against nuclear forces and C3I sites, and would be sufficiently selective not to provoke a nuclear response.”⁷⁵ These concerns became more explicit by 2011, when senior Russian commanders highlighted potential increases in NATO's cruise-missile inventories, which could be used for “a decapitating strike … against the upper level of command and control agencies.”⁷⁶ These concerns led Russia to reorganize its air and aerospace defenses.⁷⁷

Many important Russian military installations and assets are in southern and southwestern Russia, proximate to Ukraine.⁷⁸ These include nuclear weapons storage sites, long-range aviation bases, air defense bases, and a missile base. These are systems that Russian doctrinal publications have regularly described as principal interests. The south-facing Armavir ballistic missile early-warning radar system is less than 400 kilometers from the Ukrainian coastline of the Sea of Azov. Ukraine attacked this system with drones in May 2024.⁷⁹ Other such radars in western Russia are already vulnerable to short-warning attacks from NATO territory.⁸⁰ As events in the Russo-Ukrainian War demonstrate, Moscow itself is within drone range of Ukraine, and even relatively low technology designs could penetrate Russian air defenses.⁸¹ More pointedly, modern long-range stealthy cruise missiles, such as the German-Swedish Taurus, easily have the speed and range to accurately attack many of these targets in an hour or less.⁸² A short-range ballistic missile, such as the U.S. Army's new “precision strike missile” that has a range of over 400 kilometers and is compatible with the Multiple Launch Rocket System/Army Tactical Missile System launch vehicles currently in service, could strike these targets in under ten minutes.⁸³ It is noteworthy that after Russia annexed Crimea in 2014, it promptly moved in major air defense systems, improving its strategic air defense capability.⁸⁴

The Russians would also have been concerned about the prospect of NATO intelligence-gathering installations in Ukraine. U.S. and NATO signals intelligence (SIGINT) installations in Norway have long been a feature of that country's membership in NATO, despite Norway's long-standing unilateral commitment not to allow either permanent NATO bases or NATO nuclear weapons in its territory.⁸⁵ Russia moved new SIGINT installations into Crimea after seizing it.⁸⁶ One can infer from these moves that Russia would be deeply concerned if NATO membership for Ukraine were to produce the reverse possibility for NATO.

Finally, Russia is particularly concerned about the Black Sea.⁸⁷ Russia has historically seen the Black Sea as providing strategic depth, which helped to ensure the defense of southwestern Russia. A RAND assessment of the security competition in the Black Sea region states baldly that Russian leadership would view NATO membership for Ukraine and Georgia as “unacceptable and a direct threat to Russia's national security.”⁸⁸ Moreover, the report notes that “Southwestern Russia includes the vast Volga River Basin, where 40 percent of Russia's residents live, and which accounts for 50 percent of the country's agriculture and 45 percent of its industrial production, including major segments of the defense industry.”⁸⁹ Given that Turkey (1952), Bulgaria, and Romania (both 2004) were already members of NATO, the membership promise in 2008 to Ukraine and Georgia, once it reached fruition, would essentially make the Black Sea a NATO lake. Since 2019, the United States has been helping Ukraine upgrade the Ochakiv naval base in Mykolaiv oblast (region), not far from Odessa, to be able to service NATO warships.⁹⁰ Putin himself called out a U.S. B-1B Lancer bomber exercise in the Black Sea, which either coincided with, or was part of, an annual NATO nuclear deterrence exercise, “Steadfast Noon,” in southern Europe in 2021.⁹¹

Critics of Putin's decision-making, and those persuaded that his motivations for war were nonstrategic and ideological, are quick to assert that it was improbable that Ukraine would ever actually be invited to join NATO. The question, however, is how Russia assessed this possibility, not how outside observers assessed it. Russia had plenty of evidence that NATO membership for Ukraine was not only probable but increasingly likely.

The background of NATO's 2008 promise of future membership to Ukraine would alone have given the Russians pause. While U.S.-Russian relations were far from ideal then, there was little by way of a concrete Russian threat that drove the U.S. proposal.⁹² Indeed, even the Ukrainians were not clamoring for membership in 2008.⁹³ In her memoir, former Secretary of State Condoleezza Rice does not explain why George W. Bush decided to extend NATO membership to Ukraine.⁹⁴ It just somehow happened. In his memoir, Ambassador Burns says that he thought that the membership initiative was “a strong legacy building effort.”⁹⁵ Several NATO allies, including Germany and France, thought extending NATO membership to Ukraine was a bad idea.⁹⁶ The Bush administration, assisted by the United Kingdom, cajoled these powers into accepting a pledge of eventual membership without specifying a date.

From the Russian point of view, this project was thus problematic for two reasons: The United States would continue to push NATO's frontiers forward, and the allies in Europe would, regardless of their own preferences, succumb to U.S. blandishments. Not long after the NATO summit, in the last month of the Bush administration, the United States and Ukraine signed the U.S.-Ukraine Charter on Strategic Partnership (December 19, 2008). Within months of taking office, the Barack Obama administration and Ukrainian President Victor Yushchenko established the Strategic Partnership Commission to realize the charter's goals. “The Charter also emphasizes the continued commitment of the United States to support enhanced engagement between NATO and Ukraine.”⁹⁷ It is unlikely that the Russians were unaware of all this activity.

The EU developed its own plans to further integrate Ukraine into Western Europe in 2014. Before then, the Russians had relied on a corrupt and pliant Ukrainian regime, and sectional political divisions in the society, to keep Ukraine in a muddy middle ground between the West and Russia. Putin tried to outbid the EU to prevent the first steps on the path to EU membership; he succeeded, but escalating political violence over the matter in Ukraine ended with Ukrainian President Viktor Yanukovych fleeing.⁹⁸ A much less Russia-friendly, more nationalistic Ukrainian government succeeded him. Russia promptly seized Crimea—perhaps out of pique and perhaps out of preventive motives of the kind discussed here. Putin then used proxy violence in the Donbas to create two secessionist oblasts, Donetsk and Luhansk, ultimately leading to a frozen conflict.

The Minsk agreements of 2014 and 2015 (also known as Minsk I and Minsk II) aimed to stop and settle politically the secessionist war (and Russian intervention) in the Donbas.⁹⁹ Ukraine, Russia, the OSCE, and the secessionist leaders of Donetsk and Luhansk negotiated these agreements. Germany and France played a particularly important role. (The United States was not a participant.) Combat in the Donbas was going badly for Ukraine in this period, and one purpose of the diplomacy was simply to avert military collapse. Aside from attempting to stop the fighting and separate the warring parties, the two agreements included complicated provisions to resolve the conflict that would bring the Donbas back under a semblance of Ukrainian government authority, while granting the two oblasts a high degree of autonomy. It was, of course, the weakness and ambiguity of these provisions that made the agreements attractive to Moscow. Critics of the agreement, including many Ukrainian politicians, did not support the settlement's autonomy provisions. In their view, Russia-friendly elements in the secessionist republics would have so much scope for political mischief inside Ukraine that integrating with the West, including EU and NATO membership, would be impossible.¹⁰⁰ In August 2015, violent riots erupted in Ukraine to oppose the agreements. The Minsk agreements were never fully implemented—each side blames the other for failing to do so.¹⁰¹ That said, even a moderately attentive Russian security establishment would have noticed something else happening—tightening relations between Ukraine and the United States and NATO. After the 2022 Russian full-scale invasion, German and French leaders declared that they had sponsored the Minsk agreements to buy time in order to improve Ukrainian military power.¹⁰²

After NATO's 2008 promise of future membership to Ukraine, Ukrainian leaders and their partisans abroad pushed to initiate a MAP. Given that Ukraine and its supporters campaigned intensely to start a MAP, and that some member states opposed doing so with equal intensity, observers would be forgiven for thinking that Russia should have taken some comfort from the delay. But this is true only to a limited extent.

NATO created the MAP as a post–Cold War attempt to nudge new applicants to conform to existing members’ governance, civil-military, and military institutions and practices.¹⁰³ A MAP is neither a prerequisite for nor a guarantee of NATO membership. Moreover, the standards that the MAP imposes are rather loose. The MAP process is not a part of the NATO treaty. It is simply a process that NATO cooked up to achieve some of its objectives. There was no MAP process for Sweden and Finland. Instead, NATO members simply agreed: Sweden and Finland are like us, so a MAP is unnecessary.

The MAP was created because many of the post–Cold War applicants to NATO had been nondemocratic members of the Soviet-controlled Warsaw Pact military alliance or actual republics of the Soviet Union. Existing members of the alliance did not wish to import their illiberal Soviet-era practices and problems into the alliance. The MAP expected applicants to adhere to significant reforms, including a series of tutorial arrangements in which NATO oversight committees would help implement the new standards.¹⁰⁴ It was implied that failure to meet these standards would delay membership. In fact, there never was a clear yardstick for “passing” the MAP test. States were brought into NATO without report cards, at least without public ones.¹⁰⁵ Indeed, NATO relaxed some of the MAP criteria for certain member states during NATO's Big Bang enlargement in 2004, when it admitted seven new member countries.¹⁰⁶

That said, Ukrainian leaders and Ukraine's friends abroad, including states, nongovernmental organizations, and independent experts, lobbied hard for a MAP. Because many of the same member states who were initially leery of membership for Ukraine remained leery, and because the Russians unequivocally opposed NATO membership for Ukraine, the MAP process was perpetually postponed. From a Russian point of view, this was a small victory. But the victory simply slowed rather than stopped Ukraine's progress toward membership, further obfuscating the process.

NATO's own website enumerates a history of dense and growing ties with Ukraine going back to the dissolution of the Soviet Union. The Russian mission to NATO would have directly observed these ties. Perhaps more importantly, these dense and growing ties conformed closely to what NATO would have asked of Ukraine as part of a MAP.¹⁰⁷ Put another way, NATO was helping Ukraine position itself such that it could join NATO quickly, as soon as the alliance decided to admit it

In 1997, NATO and Ukraine signed the Charter on a Distinctive Partnership, which, as the title suggests, was a largely political document, though it mentioned reforms that NATO would have asked of Ukraine in a MAP.¹⁰⁸ It further established a NATO-Ukraine Commission to manage the relationship. In 2002, NATO and Ukraine agreed on the NATO-Ukraine Action Plan, to “identify clearly Ukraine's strategic objectives and priorities in pursuit of its aspirations towards full integration in Euro-Atlantic security structures.”¹⁰⁹ The document details political, economic, and military steps that mirror those that would be in a MAP, including creating processes and institutions to annually assess Ukraine's progress.¹¹⁰ Seven years later, Ukraine and NATO issued the Declaration to Complement the Charter on a Distinctive Partnership to support NATO's 2008 Bucharest Summit agreement that Ukraine would ultimately be invited to join NATO.¹¹¹ As with the 2002 NATO-Ukraine Action Plan, the NATO-Ukraine Commission oversaw how to implement this declaration. This process appears identical to the 2002 effort and indeed probably simply absorbed it.¹¹²

Some Ukrainian and Georgian observers viewed the “Annual Target Plan” process as essentially a “substitute for the MAP” and understood that NATO could decide about membership at a later date without any MAP process at all.¹¹³ Russian observers also viewed the Annual Target Plan process as a shadow MAP. In the words of Dmitrij A. Danilov, “Since an invitation to MAP does not guarantee quick accession in any case, Ukraine moves toward the desired aim outside this program so that to negotiate the necessary formal procedures as soon as the political situation makes this possible. For Ukraine, this is a kind of common-law marriage in expectation of the marriage contract. NATO, on its side, is seeking stronger ties without committing itself.”¹¹⁴ In May 2022, the interim U.S. chargé d'affaires in Ukraine, Kristina Kvien, confirmed this perception: “On the subject of Finland's accession to [NATO] without a Membership Action Plan. It is indeed so that the Action Plan is not obligatory for joining NATO. Accordingly, Ukraine could also join the Alliance without it. I wouldn't consider it to be part of an expedited procedure as the Action Plan is a mechanism that enables countries to meet the membership requirements. If a country already meets them, then there is no need for it.”¹¹⁵ This was effectively U.S. policy. When the U.S. Senate Foreign Relations Committee asked Secretary of State Anthony Blinken, “Does this administration still support a membership action plan for Ukraine and for Georgia?” he responded: “We support Ukraine membership in—in NATO. It currently has all of the tools it needs because since the membership action plan was created, a number of other important tools were developed to help countries prepare for possible NATO membership including an annual program that Ukraine benefits from. In our estimation, Ukraine has all the tools that it needs to continue to move forward in that direction.”¹¹⁶

Table 1 captures many of the high points of the NATO-Ukraine relationship. It is not meant to fully represent the political events that may have precipitated or followed any of these activities. Rather, it is meant to illustrate how Russian observers may have perceived these developments between Ukraine and the West. From 1998 to 2021, Russia had a diplomatic mission to NATO. Despite the ebbs in Ukrainian and NATO amity and efforts, and hesitance among some NATO members to take the last step of offering Ukraine membership, Russia saw a seemingly inexorable process of ever closer linkages to NATO. In Russia's view, NATO could ratify accension protocols for Ukraine in as brief an interval as it ultimately did for Finland in 2023. That effort took one year, but half that time was to assuage Turkey, who opposed the move. Moreover, these various initiatives were not merely bureaucratic formalities—Ukraine's military and NATO became tightly interconnected.

These steps in Ukraine's growing relationship with NATO have been institutionalized in committees, working groups, and missions to Ukraine, as noted above. But more importantly, Ukraine's armed forces have been cooperating more with both NATO itself and NATO member states. Specifically, the Ukrainian military and NATO conducted joint exercises to ensure interoperability and improve Ukrainian combat power. The White Books of the Ukrainian Ministry of Defence chronicle nearly 130 multinational exercises (some small, some large) from 2010 to 2020, most involving NATO countries.¹¹⁷ I outline three examples of the larger exercises below.

First, since 1997 Ukraine and the United States have jointly hosted Exercise Sea Breeze, in the Black Sea.¹¹⁸ Formally, Exercise Sea Breeze is not a NATO exercise. But over the years, Exercise Sea Breeze has involved so many alliance participants that it is virtually indistinguishable from a NATO exercise. Sea Breeze itself evolved from relatively innocuous exercises in counter-piracy and maritime policing into full spectrum training in all aspects of naval warfare. The operations’ pace and complexity intensified after Russia annexed Crimea, and by 2021, Sea Breeze involved thirty-two countries and forty ships.¹¹⁹

Second, Fearless Guardian, an effort to train Ukrainian ground forces, began in 2015 as a U.S.-Ukraine effort.¹²⁰ The United States and Ukraine established the Joint Multinational Training Group-Ukraine (JMTG-U) at the Combat Training Center in Yavoriv (an early target of Russia bombing in the 2022 invasion). U.S. Army training groups participated in nine-month rotations to the center to teach combined arms skills and improve interoperability with NATO. Since 2015, Canada, Denmark, Lithuania, Poland, Sweden, and the United Kingdom have participated in this effort.¹²¹

Third, Exercise Rapid Trident commenced in 2006 under the auspice of NATO's Partnership for Peace. Also centered at Yavoriv, it evolved into “a validation exercise for Ukrainian armed forces undergoing training with NATO allies.”¹²² It is unlikely that Russian intelligence missed any of these developments. Aside from the standard tools of satellite imagery and signals intelligence, it is generally believed that the Russians had considerable success with human intelligence efforts in Ukraine.¹²³

I have argued thus far that theory, historical practice, and specific evidence suggest that Russia's concerns about future NATO membership for Ukraine were one of the primary motives for Russia's 2022 offensive. Putin, and many in the Russia foreign policy establishment, likely feared that Ukraine's membership in NATO would change the balance of power. Putin and his advisers also feared, on the basis of the post–Cold War record and of accelerating Ukrainian-NATO ties during Volodymyr Zelensky's presidency, that the odds of NATO membership for Ukraine were going up, not down, and at an accelerating rate.

The same intensifying relations that would have made NATO membership seem more likely also had practical consequences: Ukrainian military power was improving. Consequently, Russia faced a closing window of opportunity to do anything militarily about Ukraine's trajectory. In an oral history on the Russian invasion, Haines argued, “He [Putin] saw Ukraine inexorably moving towards the West and towards NATO and away from Russia, … He saw the Ukrainian military becoming significantly stronger…. And if you were looking at it through the lens of somebody who perceived Ukraine moving away from Russia as being something that you had to stop at all costs, you could begin to see how it wasn't going to get any easier over time.”¹²⁴ According to the U.S. intelligence community, an important element of Putin's calculus was that he and at least some of his advisers saw a closing window of opportunity.¹²⁵

After Russia annexed Crimea and sponsored secessionists in the Donbas in 2014, Western assistance to Ukraine intensified. How might Russia have assessed Ukraine's growing military capabilities and its future trajectory?¹²⁶ It is easier to collect public data on training inputs in Ukraine than it is to assess how such training affected Ukraine's military capability. Ukraine's performance since the Russian attack in February 2022 suggests that the training was quite effective. Ukrainian Ministry of Defence White Books list dozens of tactical and operational training events every year for every service. These include command post exercises, field training exercises, small unit training, and live-fire exercises.¹²⁷ The purpose of many of these exercises was not only to improve Ukraine's combat power but also to ensure that its military practices conformed with NATO standards. Under Operation Orbital, the United Kingdom helped train over 22,000 Ukrainian troops from 2015 until the Russian invasion. A UK special defence adviser was also embedded within the Ukrainian Ministry of Defence.¹²⁸ In 2021, Ukraine participated in at least twenty-one multinational training exercises, eight in or near Ukraine and thirteen abroad, most of them NATO exercises or with NATO member states.¹²⁹

Financial assistance to Ukraine was also forthcoming after 2014. According to the Congressional Research Service, “even prior to the start of the war, Ukraine was a leading recipient of U.S. military aid in Europe and Eurasia. From 2014, when Russia first invaded Ukraine, through March 2022, the United States committed more than $4 billion in State Department- and DOD-funded security assistance ‘to help Ukraine preserve its territorial integrity, secure its borders, and improve interoperability with NATO.’”¹³⁰ These funds helped Ukraine acquire U.S. equipment, much of it from U.S. Department of Defense stocks, to improve the combat power of the Ukrainian military. Much of the equipment aimed to improve Ukraine's intelligence and reconnaissance capability, electronic warfare capability, and command and control. In 2018, the United States began to ship Javelin anti-tank missiles and launchers to Ukraine, though initially Ukraine agreed not to use the weapons in combat. That year Ukraine also benefited from unspecified amounts of support from various NATO trust funds.¹³¹

In June 2020, NATO recognized Ukraine as its sixth Enhanced Opportunities Partner, a status that may have prompted Exercise Joint Endeavour, the unusually large addition to Ukraine's extant exercise schedule.¹³² Eight thousand troops from Canada, Ukraine, the United Kingdom, and the United States participated in 2020. As part of the exercise, the British Army dropped 250 paratroopers into an exercise area in what appears to be Krivoy Rog, Dnipropetrovsk Oblast, in southeastern Ukraine.¹³³ A photo on the website for U.S. European Command depicts two Ukrainian fighter aircraft escorting a B-52 strategic bomber, which is a nuclear capable aircraft, as they fly over “the Ukraine.”¹³⁴ A total of 12,500 troops from fifteen countries participated in 2021, which generated activity all over the country, including in the Black Sea and the Sea of Azov.¹³⁵ Neither NATO nor the United States claim parentage of this exercise, though it is prominently discussed in the Ukrainian Ministry of Defence White Books.¹³⁶ The exercise was integrated with Rapid Trident, which is an official NATO training exercise in Ukraine. The striking thing about Exercise Joint Endeavour is its explicit focus on defense of Ukraine. Moreover, officers from Ukraine's alliance partners directly participated in the Ukrainian command structure, including alliance combat units practicing side by side with Ukrainian ground combat units in realistic scenarios related to defending Ukraine.¹³⁷ Canadian fighter aircraft based in Romania also participated; they would have been operating over Ukraine under NATO command and control, with nodes in Germany, Romania, and Spain.¹³⁸

These exercises followed the largest ever iteration of Exercise Sea Breeze, which addressed “amphibious warfare, land maneuver warfare, diving operations, maritime interdiction operations, air defense, special operations integration, anti-submarine warfare, and search and rescue operations.”¹³⁹ Ukrainian forces were improving not only their own combat skills but also their ability to operate seamlessly with NATO forces.¹⁴⁰ And NATO forces were practicing to defend Ukraine, which the Russians would likely have seen as a harbinger of things to come. Russian observers were aware of these various exercises, citing them as evidence of the problem that NATO posed for Russian security: “Russia used exercises in Ukraine (including Joint Effort 2020 and Rapid Trident 2020) to argue the case that it was surrounded by enemies.”¹⁴¹

Putin and his advisers feared that membership for Ukraine was only a matter of time, and not that much time. NATO promised Ukraine future membership in 2008; Ukraine participated in a growing number of NATO activities during both the Donald Trump and Biden administrations; and Ukraine and its foreign partisans, both governmental and nongovernmental, seemed determined to carry the process forward. Furthermore, Russian observers were unconvinced that not having a MAP would prevent Ukraine from joining NATO. Ukrainian policymakers and policy analysts acknowledged that NATO-Ukraine cooperative efforts covered everything that a MAP would have covered. Ukraine's military integration with NATO would have made formal NATO membership a short process. Indeed, Ukraine's military was already integrated in NATO: It had trained with NATO officers, adopted NATO doctrine, and so far as one can tell, connected its air defense and intelligence systems to those of NATO. The course of the war since February 2022 suggests that this was the case. It was reasonable for Russia to fear that Ukraine could achieve formal NATO membership faster than Russia could respond. The web of political and military connections between Ukraine, NATO, and NATO member states would have made responding even more militarily costly for Russia than the current war because NATO troops could easily have been involved from the outset. Putin and the Russian establishment understand NATO to be a nuclear alliance. During the war, despite NATO's substantial military support to Ukraine, Putin has painstakingly avoided deliberate military incursions into NATO territory; he is clearly afraid of something. To be able to preempt a membership process, without running afoul of nuclear escalation risks, Russia would have to mobilize tens of thousands of troops on Ukraine's borders—and leave them there. Maintaining this continuous level of high readiness for war might not have seemed sustainable. In any case, launching such a war at the moment of membership for Ukraine would have been even riskier than the war that Russia did ultimately undertake.

Finally, Russia faced a Ukrainian military that was improving its combat power with Western advice, training, and weaponry. The costs and risks of forestalling NATO membership militarily were growing, even if NATO did not fight directly on Ukraine's side. The purpose of NATO's assistance to Ukraine was to increase Ukraine's military power in order to enhance deterrence of any future threat to Ukraine. As scholars of the security dilemma observed long ago, it is not unusual in international politics for a state to threaten others when it takes steps to defend itself.¹⁴² It is not always easy to distinguish between defense and offense. This worked in an indirect way in Ukraine. The problem was not that Ukraine would get so strong that it could threaten Russia; the problem was that Ukraine could get so strong and so integrated with the NATO military structure that Russia could no longer prevent it from joining NATO, or at least not prevent it from doing so without great cost and risk.

With some exceptions, journalists, policy analysts, politicians, and scholars discount the strategic explanation for Russia's invasion of Ukraine advanced above.¹⁴³ Instead, they stress one or sometimes all of the following: Putin's professional, political, and personal biography; an ideology present in the Russian elite that blends an unusually chauvinistic strain of Russian nationalism with either Soviet or czarist imperial nostalgia; and Russia's domestic political structure, which combines elements of autocracy, oligarchy, and corrupted democracy that have made diversionary wars attractive. Relatedly, some argue that Putin feared that the example of successful democracy in Ukraine would undermine his rule.

A lot of documentation or first-person testimonial evidence would be needed to assess fully the relative strength of unit-level explanations for Putin's decision-making and his preventive war motives. One hundred years after World War I, historians still analyze documents from the period and memoirs from key players to debate the causes of that war. What might be learned from documents and those who were present as to why Putin decided to invade Ukraine? Was it paranoid nostalgia for a vast empire from his time as a KGB agent that motivated him? Did he convert to a national-imperial ideology, or did prosaic concerns to disarm domestic resistance to his personalist rule loom large in Putin's mind? Was he so concerned about Ukraine that he needed to forcibly integrate it into Russia? This kind of information will not be available for many years, if ever. Instead, one must weigh and interpret the evidence that does exist. The following facts are not in dispute: Putin was once an intelligence agent and is an authoritarian; Russians are a nationalistic people with an imperial history; Putin himself embraces nationalism either out of conviction or convenience; and Russia is not a liberal democracy. The question is whether correlation is cause. Those who advance unit-level, domestic explanations for the war ask those seeking to understand this terrible event to discount that normal great power behavior helps explain the war at least in part.¹⁴⁴ There are good reasons to doubt that their interpretations are the final word on the matter.

My purpose here is not to disprove any of the unit-level explanations, nor to critique them in detail. Rather, my purpose is to point out that they are not entirely persuasive, and the preventive war explanation deserves equal status.

First, Putin is a former intelligence professional in the KGB; most people would object to KGB agents’ moral code. Their mission was to suspect, surveille, warn, blackmail, deceive, and even to kill. Regarding domestic and foreign policy, the Russian state under Putin's rule has done all these things.¹⁴⁵ The problem with this argument is that many great powers behave ruthlessly in their foreign and security policies. They do not need to be led by former KGB officials to do so.¹⁴⁶ And as many have noted, Putin's concerns about NATO expansion, and his specific interest in Ukraine, parallel the views and policies of previous leaders who were not intelligence agents, including Yeltsin. These views have been well represented in the Russian foreign and security establishment.

Perhaps Putin's KGB background can explain what appears to have been the oddly optimistic plan to quickly win the war through a coup de main against Kyiv, followed by a coup d’état to install a pliant leader. The “special military operation” against Kyiv does resemble when the Soviets invaded Czechoslovakia in 1968 and Afghanistan in 1979.¹⁴⁷ Putin may have found the Federal Security Service's plan to be credible because it seemed familiar and came from a familiar source. But an affinity for certain tactics does not explain why a state chooses to act. Even if his security services offered a low-cost plan for a quick change of government, one still wants to know how acting against Ukraine made it to the top of Putin's agenda.

Second, Putin's garnishes his speeches with nationalist and imperialist rhetoric, and it is tempting to credit this rhetoric as the critical evidence of his motivation for war. In the words of Fiona Hill and Angela Stent, “Russia's president invaded Ukraine not because he felt threatened by NATO expansion or by Western ‘provocations.’ He ordered his ‘special military operation’ because he believes that it is Russia's divine right to rule Ukraine, to wipe out the country's national identity, and to integrate its people into a Greater Russia.”¹⁴⁸ It is difficult, however, to attribute to Putin a master plan for restoring the Russian empire.¹⁴⁹ His speeches and writings do allude to a patriotic, romantic, ethno-national story of Russia, which treats Belarusians and Ukrainians as members of a superordinate Russian nation.¹⁵⁰ He does not advance this history as an argument to bring Ukraine back into Russia, though he does, ominously, lay the groundwork for a claim to eastern portions of Ukraine with long-standing historical ties to Russia, where many Russians and Russian speakers live. Critics overlook this critical paragraph about the emergence of Ukrainian nationalism in his famous “historical unity” speech: “What can be said to this? Things change: countries and communities are no exception. Of course, some part of a people in the process of its development, influenced by a number of reasons and historical circumstances, can become aware of itself as a separate nation at a certain moment. How should we treat that? There is only one answer: with respect!”¹⁵¹

The nationalism-imperialism argument treats Russia as inherently expansionist and discounts its security motives. According to this logic, Russia should have moved against independent Ukraine when it had the greatest opportunity to do so. Instead, Russia missed at least two opportunities. Analysts agree that in late 2014 and early 2015, the Ukrainian Army was reeling from attacks by Russian proxies and barely disguised elements of the Russian military itself. It was this very weakness that prompted Ukraine's friends to advance and Ukraine to sign the Minsk agreements, even though it did not wish to do so.¹⁵² Arguably, a similar opportunity emerged in the fall of 2019 during the impeachment proceedings against President Trump. The subject of those proceedings was Trump's threat to withhold military aid from Ukraine if its government did not dig up some dirt on the business dealings of the then presidential candidate Biden's son, Hunter Biden. Ukraine declined to assist Trump. At that moment, a ruthless imperialist could have concluded that a window had opened to attack Ukraine, because Trump would not feel obligated to help those who had failed to help him.¹⁵³ If Putin had a powerful, ideological motive to annex all or some of Ukraine, it is difficult to understand why he did not act at these two moments.

Finally, the Putin regime is indeed undemocratic, and Putin has been tightening his hold on society for many years. Opponents are intimidated, assassinated, or incarcerated. But precisely because the regime has been so adept at repression, it is hard to find a good reason for it to engage in diversionary war. In general, diversionary wars are not a good idea, because a regime that is weak enough to consider such a thing is probably too weak to fare well in such a war. It is a high-risk way to garner public support. Diversionary war has fallen out of favor in the study of international relations because it makes no sense for leaders to risk the challenges of war when they are very weak domestically, and it is hard to show that it happens often.¹⁵⁴ Similarly, scholars argue that a thriving democracy in Ukraine is uniquely threatening to Putin. The odd thing about this argument is that Ukraine is supposedly uniquely threatening because it is so similar to Russia. Putin often mentions this similarity in a positive light, but critics claim that his version of history is false. In this telling, Ukraine is similar enough to Russia that democracy there would give Russian citizens such bad ideas that Putin needed to rub it out, but so different that Putin's own story of their shared history is false. This is a fine analytical line to walk. In any case, it is difficult to argue that democracy was working out so well in Ukraine that Putin had to fear the example and the contagion that it might cause.¹⁵⁵

Scholars identify personal biography, ideology, and regime type to explain foreign policy and war in many historical cases. Although they find evidence to support their hypotheses, they do not fully explain why Putin invaded Ukraine. More importantly, these causes seem more like constants of Putin's entire rule than variables. But Russian behavior has varied since Putin came to dominate Russian politics; one cannot explain variation with a constant.¹⁵⁶ By contrast, the preventive war hypothesis identifies important variables, the values of which change ever more powerfully in the direction of increasing preventive motives for war.

Realist theory predicts the phenomenon of preventive war. Historical evidence confirms that such wars happen. Theorists and Russia experts warned for years that Russia would see NATO enlargement as a threat. They also warned that Russia would be particularly concerned about Ukraine's membership in NATO.¹⁵⁷ Since 2008, NATO, the United States, and other NATO member states deliberately and consistently developed political and military connections to Ukraine. These relationships with Ukraine have intensified since Russia annexed Crimea and sponsored secessionist movements in the Donbas. It is likely that Russia acted for preventive reasons in 2014, and its attacks were certainly a fillip to the NATO-Ukraine integration effort.

After 2014, there may have been no way for Western powers to calibrate their efforts to improve Ukraine's ability to deter future Russian aggression without also threatening Russia with the prospect of a new NATO ally along nearly 2,300 kilometers of its land and sea border. But there is also little evidence that they tried to achieve such a balance. Instead, Putin likely saw in the evolving relationship exactly what Haines suggests gave the U.S. intelligence community confidence that the war would happen. NATO membership came to seem a genuine prospect; the cost to stop it militarily would only rise. It is admittedly unclear what precipitating event or series of events made Putin decide to go war.¹⁵⁸ But the presence of both long-standing and proximate causes consistent with preventive war motives are too obvious to ignore. That many do ignore them helps explain how Russia-NATO relations reached this low point.

Numerous facts support the argument that Putin and his advisers acted within a preventive war logic. They perceived that the balance of power was changing, and such a change would be strongly inimical to Russian interests. Consequently, those actors who pursued policies that would shift the balance of power are at least partially politically responsible for the war. That responsibility is even greater if they knew that Russia perceived such actions as threatening. This includes Ukraine's own leaders, but more importantly U.S. and Western European policymakers. Ukrainians cannot be faulted for wanting membership in NATO for their own security. But neither the United States nor Western Europe required Ukrainian membership for its security. Indeed, some argued that Ukrainian membership would simply be a problem. Yet many U.S. and European leaders encouraged the Ukrainians to work toward this end and helped them to do so. If this support strongly motivated Russia's preventive attack, then their political responsibility for the war suggests that ethically they should help Ukraine defend itself. But their ethical responsibility should not end there. They should be working toward a negotiated solution to this destructive war, and to a stable postwar security relationship with Russia that would render a second round less likely.

The United States is an enormously powerful actor in international politics. But U.S. leaders often fail to consider the knock-on effects of their own policies. U.S. foreign policymakers are always alert to how the United States’ behavior affects the confidence of its allies. They are attentive to how U.S. behavior does or does not strengthen deterrence of its potential adversaries. But U.S. leaders are often oblivious that U.S. power and behavior might feel threatening to other states. In the case of the United States encouraging Ukraine's Euro-Atlantic aspirations, the United States simply did not consider the possible effects on Russia. When U.S. leaders consider themselves to be right, they are disinclined to what one might call strategic empathy. This has been particularly true in the post–Cold War period, when the United States has wielded much more power than other states. This power creates a kind of moral hazard for the United States—it has the luxury of imagining that it is fully insured against all eventualities, so it does not think about them. But international politics has a way of biting the careless.

These observations are especially pertinent as the United States considers its policies in Asia. China is a bumptious rising power, and the United States has committed itself to constraining China's material power and the global influence that it perceives to be commensurate with that power. U.S. policy is understandable, even prudent. At the same time, however, the United States has chosen to make a stand on Taiwan—a particularly sensitive issue for China. The United States works assiduously to improve both its own ability to wage war in the Taiwan Strait and Taiwan's own defensive capacity. It also focuses on strengthening a web of alliance support in Asia and abroad to resist any Chinese effort to use force to settle the Taiwan matter. The United States leads the effort to close any short-term military window of opportunity that China might perceive, or perhaps ensure that no such window opens. The United States and its friends must be careful that China does not perceive efforts to improve Taiwan's ability to defend itself as a shield for Taiwan to gain independence. It would be best if there is no retrospective article ever written on China's preventive war over Taiwan.

The author thanks Michael Desch, Charles Glaser, Jonathan Shimshoni, Stephen Van Evera, and the anonymous reviewers for helpful comments on earlier drafts of this essay. MIT Security Studies Program graduate research assistants Suzanne Freeman, Jung Jae Kwon, and Wright Smith all made important contributions.