There is a rich literature on the circumstances under which the United Nations Charter or specific Security Council resolutions authorize nations to use force abroad, and there is a rich literature on the circumstances under which the U.S. Constitution and statutory law allow the President to use force abroad. These are largely separate areas of scholarship, addressing what are generally perceived to be distinct legal issues. This Article, by contrast, considers these two bodies of law together as they relate to the United States. In doing so, it makes three main contributions. First, it demonstrates striking parallels between the structure of the international and domestic legal regimes governing the use of force, and it explains how this structure tends to incentivize unilateral action. Second, it theorizes that these two bodies of law are interconnected in previously overlooked ways, such that how the executive branch interprets law in one context can be and often is informed by the other legal context. Third, it documents these interactions over time for several important components of the law on the use of force and shows that this dynamic has played a significant role in justifying the practice-based expansion of unilateral war powers. The Article concludes by arguing that both scholars and policymakers seeking to shape the law on the use of force need to take better account of this interactive dynamic.