This book explores presidential justifications of every major American military conflict from the War of 1812 to the Second Gulf War. It generates two important findings. First, presidents employ a specific standard (the Necessity Standard) publicly to justify decisions to go to war, and privately to make decisions regarding war and peace. The Necessity Standard holds that major military force should be used if no viable alternatives are available to protect vital interests or discharge duties. Second, when addressing the Necessity Standard, presidents have disclosed military and security policies that vary considerably in their patience with alternatives and their definitions of vital interests and duties. The book concludes by characterizing wars, categorizing presidential policies, and outlining how the central position of the Necessity Standard in the American politics of war and peace might affect policymaking processes, conflict management, and the public perceptions of wars and foreign policy.