Courts and scholars have vigorously debated the proper role of customary international law in American courts: To what extent should it be considered federal common law, state law, or general law? The debate has reached something of an impasse, in part because various positions rely on, but also are in tension with, historical practice and constitutional structure. This Article describes the role that the law of nations actually has played throughout American history. In keeping with the original constitutional design, federal courts for much of that history enforced certain rules respecting other nations' perfect rights (or close analogues) under the law of nations as an incident of political branch recognition of foreign nations, and in order to restrain the judiciary and the states from giving other nations just cause for war against the United States. Rather than viewing enforcement of the law of nations as an Article III power to fashion federal common law, federal courts have instead applied rules derived from the law of nations as a way to implement the political branches' Article I and Article II powers to recognize foreign nations, conduct foreign relations, and decide momentous questions of war and peace. This allocation of powers approach best explains the most important federal cases involving the law of nations across American history. This Article does not attempt to settle all questions of how customary international law interacts with the federal system. It does aspire, however, to recover largely forgotten historical and structural context crucial to any proper resolution of such questions.