Perhaps no Article I power of Congress is less understood than the power to "define and punish . . . Offences against the Law of Nations."1 There are few scholarly works about the Clause; Congress, the Supreme Court, and the Executive Branch have seldom interpreted the Clause, and even then they have done so in a cursory and contradictory manner. Relying on textual analysis and Founding-era history and political theory to read the Clause in a different manner than previous commentators, this Article seeks to rescue the Clause from obscurity and thereby enrich current foreign affairs debates. Not only is the Clause a power to civilly or criminally regulate individuals when their conduct violates customary international law-as previous commentators have assumed-but it is also a power to punish states, both foreign and U.S. states, for violations of international law. This dual meaning of the Clause-operating on both individuals and states-results from the fact that the eighteenth-century law of nations was founded on an analogy between individuals and states. Relations between states in the international system were analogized to relations between individual people in the putative state of nature-made famous by Locke, Hobbes, Rousseau, and others-where mankind allegedly lived before entering civil society. In eighteenth-century thought, not only individuals but also states were capable of committing "offences against the law of nations." And states, not just individuals, were liable to "punish" and be punished for such offenses. There are important implications of this dual reading of the Constitution's Law of Nations Clause for current debates about the constitutional status of international law and the Constitution's textual division of war and foreign policy powers between Congress and the President.
Some of the most contested issues in U.S. foreign relations law today concern whether and, if so, how international law either restrains or empowers the federal and state governments of the United States. Consider the following questions, which may seem unrelated but, as will be discussed below, raise a common constitutional issue-the proper interpretation of the Article I grant to Congress of "Power . . . [t]o define and punish Piracies and Felonies committed on the high Seas, and Offences against the Law of Nations":2
* May Article III civilian courts bind military or other federal Executive Branch officials to standards of conduct found in treaties or customary international law when, for example, they try suspected terrorists in military commissions outside the borders of the United States?3
* May the President deploy U.S. military forces abroad, without ex ante congressional authorization, to stop a foreign government's commission of genocide or ethnic cleansing in violation of treaties and customary international law?4
* May Congress direct the President to take coercive diplomatic actions against a foreign government that is committing genocide in violation of treaties and customary international law?5
* May U.S. federal courts prohibit state governments from executing persons who committed capital crimes as juveniles on the ground that doing so violates customary international law?6
Customary international law is the unwritten "law" of the international community that "results from a general and consistent practice of states followed by them from a sense of legal obligation."7 Today's customary international law is the closest modern analogue of the eighteenth-century "law of nations."8 This Article will use the term "customary international law" when discussing the present and recent past and the term "law of nations" when discussing the eighteenth-century Founding era. This unwritten international law differs in important respects from treaties. Because all states of the world, and perhaps various kinds of nonstate actors as well,9 participate in making customary international law, its substantive content can often be little controlled by the political branches of the U. …