As the United States begins to withdraw troops from Afghanistan in the coming year, courts will increasingly face the task of interpreting the dozens of federal laws whose operation depends on the existence of war. The 2009 Military Commissions Act (MCA), for instance, makes offenses triable by military commission “only if the offense is committed in the context of and associated with hostilities.” The 2001 Authorization for Use of Military Force (AUMF) empowers the President to target or detain certain individuals only “for the duration of these hostilities.” Scholars have long assumed that the determination whether or not the United States is at war is a political question, beyond the power of the courts to consider. This Article challenges that view, demonstrating that courts have repeatedly engaged such questions in statutory interpretation in conflicts past, and arguing that the temporal limits of the AUMF and MCA pose similarly justiciable questions. Yet while questions of war’s existence may be susceptible of judicial resolution, the possibility that a court’s answer may prevail over a differing view of the executive seems instinctively troubling. Separation of powers theory has long recognized interests in allocating decision-making authority among the branches so as to promote political accountability and take advantage of comparative institutional expertise. Such interests would seem to require that courts at least defer to executive determinations of war’s existence. Recognizing the constitutional salience of such interests, this Article contends that courts should only defer when doing so will in fact serve them. And those goals are not invariably advanced by deference to the executive in interpreting the AUMF and MCA. Active judicial participation in statutory interpretation may create incentives for congressional action that had previously been absent, making it harder for Congress to shirk participation in democratic debates in which the Constitution expects it to engage. Likewise, while some aspects of the existence-of-war inquiry may turn on predictive judgment, best reserved to executive expertise, other aspects of the inquiry turn on objective factual assessments – the kind the judiciary regularly undertakes. These dynamics, coupled with the equally salient separation-of-powers purpose of protecting individual rights, suggest a broader role for the judiciary in interpreting existence-of-war conditions than has been previously understood.