5 Customary International Law
This chapter considers the status in the U.S. legal system of customary international law, which was historically referred to as part of the “law of nations.” After considering what the text of the Constitution suggests about this issue, the chapter discusses how courts historically applied customary international law in cases in which it was relevant and how courts referred to it (in cases such as The Paquete Habana) as “part of our law.” The chapter also recounts the modern debates and uncertainties about the current domestic legal status of customary international law. In particular, the chapter explores the possibility that customary international law might have the status of post-Erie “federal common law” and what such a status might mean for questions of jurisdiction, preemption of state law, and limitations on congressional and executive authority. It also discusses various ways in which customary international law can be important in the U.S. legal system even if it is not applied directly by the courts, such as through the Charming Betsy canon of construction. The chapter concludes by discussing controversies concerning the Supreme Court’s consideration of foreign and international law materials when interpreting the U.S. Constitution.