When should the United States and other members of the international community intervene to stop a government from harming its own citizens? Since World War II, the main standard for intervention has been the high bar of genocide, although the international community has rarely acted to stop it. The main alternative—the “responsibility to protect”—would set the bar so low that virtually every instance of anarchy or tyranny would create unbounded obligations beyond the capacity of states to fulfill. A new standard—the pragmatic standard of humanitarian intervention—can help guide decisionmakers on when to intervene to stop governments from targeting their own citizens. The standard has three requirements: (1) an ongoing campaign of mass homicide sponsored by the government; (2) a viable plan for intervention with reasonable estimates of low casualties for the intervening forces; and (3) a workable strategy for creating lasting local security for the threatened population. The pragmatic standard was met in the recent successful intervention in Libya as well as in other cases over the last twenty years, and it should become the basis for deciding which humanitarian crises justify international intervention in the future.

The George W. Bush administration's national security strategy, which asserts that the United States has the right to attack and conquer sovereign countries that pose no observable threat, and to do so without international support, is one of the most aggressively unilateral U.S. postures ever taken. Recent international relations scholarship has wrongly promoted the view that the United States, as the leader of a unipolar system, can pursue such a policy without fear of serious opposition. The most consequential effect of the Bush strategy will be a fundamental transformation in how major states perceive the United States and how they react to future uses of U.S. power. Major powers are already engaging in the early stages of balancing behavior against the United States, by adopting “soft-balancing” measures that do not directly challenge U.S. military preponderance but use international institutions, economic statecraft, and diplomatic arrangements to delay, frustrate, and undermine U.S. policies. If the Bush administration continues to pursue aggressive unilateral military policies, increased soft balancing could establish the basis for hard balancing against the United States. To avoid this outcome, the United States should renounce the systematic use of preventive war, as well as other aggressive unilateral military policies, and return to its traditional policy governing the use of force-a case-by-case calculation of costs and benefits.