Armed interventions in Libya, Haiti, Iraq, Vietnam, and Korea challenged the US president and Congress with a core question of constitutional interpretation: does the president, or Congress, have constitutional authority to take the country to war? This book argues that the Constitution doesn't offer a single legal answer to that question. But its structure and values indicate a vision of a well-functioning constitutional politics, one that enables the branches of government themselves to generate good answers to this question for the circumstances of their own times. The book shows that what matters is not that the branches enact the same constitutional settlement for all conditions, but instead how well they bring their distinctive governing capacities to bear on their interpretive work in context. Because the branches legitimately approach constitutional questions in different ways, interpretive conflicts between them can sometimes indicate a successful rather than deficient interpretive politics. This book argues for a set of distinctive constitutional standards for evaluating the branches and their relationship to one another, and demonstrates how observers and officials can use those standards to evaluate the branches' constitutional politics. With cases ranging from the Mexican War and World War II to the Cold War, Cuban Missile Crisis, and Iran-Contra scandal, the book reinterprets central controversies of war powers scholarship and advances a new way of evaluating the constitutional behavior of officials outside of the judiciary.