While the current Administration has largely abandoned claims of plenary presidential authority to fight the nation’s wars, courts, scholars, and policy makers continue to debate the nature and scope of the powers conferred by the September 18, 2001 Authorization for Use of Military Force. This Article examines primarily Supreme Court precedent to distill the general scope and limits of the President’s powers to fight the nation’s international and non-international armed conflicts. It concludes that the Supreme Court has expressly endorsed and consistently observed (although inconsistently applied) two concepts of necessity attributable to the Commander-in-Chief power. The first is military necessity: the power to employ all military measures not prohibited by applicable law and reasonably calculated to defeat a national enemy. The case law is reasonably clear that “applicable law” in this context includes all domestic and international law specifically applicable to armed conflict. Military necessity also encompasses a second type of necessity: public necessity. Analogous to the common law tort doctrine, precedent reveals that, in armed conflict, public necessity permits the abrogation of private, statutory, and even certain constitutional rights under sufficiently exigent circumstances. A third necessity related to war has been theorized but never clearly addressed by the courts: an alleged presidential power to take all actions necessary to counter an imminent threat to the nation’s existence. This is best understood as an extreme form of public necessity, here termed “governmental necessity.” This Article distills and relates these three forms of necessity, explaining how they inform and complicate questions regarding the President’s powers to conduct war.