International courts play a small but growing role in resolving interstate disputes as well as in directly applying criminal law to individuals. The United States’ participation in such tribunals raises an array of serious constitutional questions—questions that are particularly urgent in light of the United States’ potential accession to the International Criminal Court (ICC), for this court would have jurisdiction over U.S. nationals and U.S. territory. This Article presents perhaps the best source of understanding these questions: the discussion of the constitutionality of joining international courts to try slave traders in the early nineteenth century. This obscure episode has never before been examined for its relevance to current constitutional debates.

The permissibility of the United States joining international tribunals spans two major constitutional issues: the vesting of judicial power in non–Article III courts and the delegation of federal powers to supernational institutions through the treaty power. Both areas of law are known for their zigzagging lines of cases and contentious academic debate. Nonetheless, the preponderance of scholarly opinion concludes that the Constitution permits U.S. participation in the ICC and similar tribunals.

The literature on these questions, however, has neglected an important precedent. In the wake of the Napoleonic Wars, Britain created a network of international tribunals to punish slave trading. Many European nations joined these “mixed courts.” The United States refused to participate, arguing that the Constitution forbade joining an international criminal court with jurisdiction over American nationals. The constitutional objections were formulated by some of the leading statesmen of the early Republic, including some members of the founding generation. As with the ICC, an initial rebuff by the United States did not end efforts to secure U.S. membership. Still, the United States stayed aloof from the mixed-court system for forty-five years—a period spanning eleven presidencies with varied politics and attitudes toward Britain, international engagement, and the slave trade.