This Article examines the unique interpretive and constitutional challenges presented by recent maritime piracy cases in the United States.  Federal law defines piracy by reference to the law of nations, or customary international law.  Because customary international norms may change over time, the U.S. piracy statute effectively codifies a dynamic definition of the criminal offense into domestic law.  Federal courts historically have ascertained the substantive content of norms of customary international law through an empirical enterprise: examining state practice and opinio juris sive necessitatis.  However, in recent years federal courts have instead deferred to the Executive’s view that article 101 of the U.N. Convention on the Law of the Sea is a codification of piracy jure gentium.  In the process, some courts have treated the treaty provision as a current, authoritative, and exhaustive definition of piracy.  It is argued that this approach is deeply problematic, not least because it has led courts to prioritize the text of a treaty to which the United States is not a party over actual state practice and opinion juris. This Article therefore recommends a two-step approach to ascertaining the meaning of piracy jure gentium that gives all deference due to the Executive without abdicating the essential function of the judiciary in saying what the law—in this case, international law—is.

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