The U.S. Supreme Court's most momentous case for customary international law in the early twentieth century – and perhaps in its entire history – involved a man injured while walking home along a railroad track in Pennsylvania. Erie Railroad Co. v. Tompkins, a wholly domestic tort case decided in 1938, fundamentally changed the way federal courts think about common law. In doing so, it had important yet inscrutable implications for how federal courts should think about customary international law – implications that remain debated to this day. The story of the Supreme Court and customary international law in the early twentieth century is, therefore, to a large extent a story about Erie.

That is so as well because in other respects the early twentieth century was not a fertile period for the direct application of customary international law in U.S. courts. Many staples of international law adjudication in the nineteenth century – pirates, prizes, and privateers – faded or disappeared altogether. The United States of course played an increasingly prominent role on the world stage through its acquisition of overseas territory, its participation in two world wars, and its engagement with an increasingly transnational economic system. But for various reasons these events did not produce an extensive record of the direct application of customary international law. Among other things, the period saw a rise of treaties and statutes, often reducing customary international law's role to a supporting one as a tool of interpretation.