The Power to Define Offenses Against the Law of Nations

Congress has the power to “define and punish...Offenses against the Law of Nations.” This clause clearly empowers Congress to punish universally recognized offenses under international law, piracy being one obvious example. But Congress also has the power to “define” offenses against the law of nations. Surely punishing an offense presupposes defining it. So what does “define” add?This paper provides an answer. The Constitution’s text and structure, early constitutional history, and modern foreign relations doctrine all suggest that Congress has the power to define offenses against the law of nations that preexisting international law does not proscribe. Congress may pass laws prohibiting private conduct that violates international law, as well as any private conduct that the United States has an international duty to punish. It can also punish offenses if it is ambiguous whether international law requires it too. Congress probably even has the power to create new offenses in order to foster changes in customary international law.