After over thirteen years of continuous armed conflict, neither courts nor scholars are closer to a common understanding of whether, or how, international and U.S. law interact to regulate acts of belligerency by the United States. This Article articulates the first normative theory regarding the relationship of customary international law to U.S. domestic law that fully harmonizes Supreme Court precedent. It then applies this theory to customary international laws of war to better articulate the legal framework regulating the armed conflicts of the United States. It demonstrates that the relationship of customary international law to U.S. law differs in cases involving war and other exercises of “external” sovereign powers from cases involving “internal” sovereign powers. In cases involving matters external to the sovereignty of the United States, including the exercise of external sovereign powers of war, the Supreme Court traditionally treated customary international law as a form of external, positive law, and applied it as an exogenous, non-federal rule of decision in accordance with conflict-of-laws principles. The Court articulated its “external” choice-of-law framework in Paquete Habana: “where there is no treaty, and no controlling executive or legislative act or judicial decision, resort must be had to the customs and usages of civilized nations.” The Article then analyzes the Court’s wartime jurisprudence to more thoroughly explicate the Paquete Habana framework in the nation’s armed conflicts, explaining the relationship of international laws of war to the Constitution and laws of the United States. This analysis not only confirms the Article’s general customary international law thesis but also clarifies important implications of the Court’s use of international law as an exogenous rule of decision, particularly, that such rules need not be entirely consistent with the Bill of Rights. Given the range of issues this Article clarifies, it should significantly influence academic and judicial discourse regarding the relationship of customary international to U.S. law generally, and especially in cases involving the armed conflicts of the United States.