A decade of war has meant a decade of writing on war powers. From the authority to start a war, to restrictions on fighting wars, to the authority to end a war, constitutional lawyers and scholars have explored the classic issues (war initiation, prosecution, and termination) through the classic prisms (text, history, and function) for a new generation of national security challenges. Despite the volume of writing on war powers and the urgency of the debates in the context of Iraq, Afghanistan, Libya, and Syria, war powers debates are widely seen as stagnant. We introduce a new set of perspectives into the war powers literature. Over the last four decades, behavioral psychologists have identified persistent biases in individual and group decisionmaking. The behavioral revolution has had a significant impact on legal scholarship—primarily in law and economics—and has also influenced scholars in international relations, who increasingly write about psychological biases and other decisionmaking challenges. These insights, however, have yet to be applied in the war powers context. This Article brings the behavioral literature into the conversation on war powers, showing how lessons from behavioral psychology are relevant to decisions on war and peace. It outlines a variety of psychological biases that bear on decisions about war and peace, applies these lessons to a variety of war powers debates, and discusses broader institutional design strategies for debiasing decisionmaking. The lessons of psychology provide new functional perspectives on classic war powers debates: the authority of Congress versus the President to initiate wars, the scope of presidential authority to use force, the ability of Congress to restrict the conduct of war, the War Powers Resolution and the termination of wars, and the role of the United Nations. Some of the decisionmaking biases point in conflicting directions, so there are no simple answers or tidy solutions. But understanding where important decisions risk going wrong is the first step in figuring out how to make them go right.