Courts and scholars continue to debate the status of customary international law in U.S. courts. The modern position argues that courts should treat customary international law as federal common law. The revisionist position contends that customary international law applies only to the extent that positive federal or state law has adopted it. Neither approach, however, adequately takes account of the Constitution’s allocation of powers to the federal political branches or the judiciary’s treatment of the law of nations throughout U.S. history. The Constitution’s allocation of specific powers to the federal political branches—such as the powers to recognize foreign nations, declare war, issue letters of marque and reprisal, and make rules governing captures—can only be understood by reference to certain background principles of the law of nations. Many of the Supreme Court’s decisions applying traditional principles derived from the law of nations may plausibly be understood as upholding the Constitution’s allocation of war and foreign relations powers to the political branches of the federal government. In numerous cases, the Court has upheld the rights of foreign sovereigns in ways that respect the political branches’ possession or exercise of these specific constitutional powers. This understanding has potential implications for the ongoing debate over the status of customary international law in U.S. courts. Specifically, it suggests that the modern position is over-inclusive and the revisionist position is under-inclusive of the role of customary international law. The allocation of powers approach—grounded in specific constitutional provisions and supported by numerous Supreme Court cases—suggests that courts should apply the law of nations when necessary to uphold the Constitution’s exclusive allocation of war and foreign relations powers to the federal political branches.
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