The U.S. National Security Strategy of 2002 and the Iraq War that has followed rejuvenated a very old debate about whether a state has the right to use force preemptively, or in advance of an armed attack. Beneath the well-covered ground of preemptive, or anticipatory, self-defense, lurks an often overlooked but fundamental question: What deference, if any, should third parties–whether other states, international organizations, or tribunals–pay to the judgments made by self-defense claimants? Is discretion available to state decisionmakers who perceive threats to their nation that, they believe, can only be met with force–and cannot await collective action? Rather than addressing the doctrines of self-defense in comprehensive detail, as many others have done, this Article asks how the international community, sometimes as judge and sometimes as jury, should adjudicate claims to the exercise of self-defense. Part I addresses the basic currents underlying how international lawyers conceive of use of force claims, focusing on two aspects of international jurisprudence: the strict rule-oriented approach to self-defense and the related opposition to state discretion in the use of force, and the international legal theory of “objective responsibility.” Part II explores self-defense decisionmaking, looking at the role of perceptions and intentions in the resort to force and how the law governing self-defense generally accounts for subjective factors. Part III draws out the principles of an adjudicative approach that preserves the legal quality of self-defense assessments while taking into account the reality of threats and decisionmakers’ reasonable perceptions of them. It seeks an appropriate way to cabin subjective elements so that they do not swallow the rules themselves. A conclusion addresses some implications of this Article’s approach.