In 2013, the U.S. government announced that its nuclear war plans would be “consistent with the fundamental principles of the Law of Armed Conflict” and would “apply the principles of distinction and proportionality and seek to minimize collateral damage to civilian populations and civilian objects.” If properly applied, these legal principles can have a profound impact on U.S. nuclear doctrine. The prohibition against targeting civilians means that “countervalue” targeting and “minimum deterrence” strategies are illegal. The principle of distinction and the impermissibility of reprisal against civilians make it illegal for the United States, contrary to what is implied in the 2018 Nuclear Posture Review, to intentionally target civilians even in reprisal for a strike against U.S. or allied civilians. The principle of proportionality permits some, but not all, potential U.S. counterforce nuclear attacks against military targets. The precautionary principle means that the United States must use conventional weapons or the lowest-yield nuclear weapons that would be effective against legitimate military targets. The law of armed conflict also restricts targeting of an enemy's leadership to officials in the military chain of command or directly participating in hostilities, meaning that broad targeting to destroy an enemy's entire political leadership is unlawful.

Numerous polls demonstrate that U.S. public approval of President Harry Truman's decision to drop the atomic bombs on Hiroshima and Nagasaki has declined significantly since 1945. Many scholars and political figures argue that this decline constitutes compelling evidence of the emergence of a “nuclear taboo” or that the principle of noncombatant immunity has become a deeply held norm. An original survey experiment, recreating the situation that the United States faced in 1945 using a hypothetical U.S. war with Iran today, provides little support for the nuclear taboo thesis. In addition, it suggests that the U.S. public's support for the principle of noncombatant immunity is shallow and easily overcome by the pressures of war. When considering the use of nuclear weapons, the majority of Americans prioritize protecting U.S. troops and achieving American war aims, even when doing so would result in the deliberate killing of millions of foreign noncombatants. A number of individual-level traits—Republican Party identification, older age, and approval of the death penalty for convicted murderers—significantly increase support for using nuclear weapons against Iran. Women are no less willing (and, in some scenarios, more willing) than men to support nuclear weapons use. These findings highlight the limited extent to which the U.S. public has accepted the principles of just war doctrine and suggest that public opinion is unlikely to be a serious constraint on any president contemplating the use of nuclear weapons in the crucible of war.