The Political Branches and The Law of Nations

In the late eighteenth and early nineteenth centuries, the U.S. Supreme Court went out of its way to follow background rules of the law of nations, particularly the law of state-state relations. As we have recently argued, the Court followed the law of nations because adherence to such law preserved the constitutional prerogatives of the political branches to conduct foreign relations and decide momentous questions of war and peace. Although we focused primarily on the extent to which the Constitution obligated courts to follow the law of nations in the early republic, the explanation we offered rested on an important, but largely overlooked, predicate - that the political branches were free to make law in derogation of the law of nations, and that such law would bind U.S. courts as the supreme law of the land. In this Article, we explain how Supreme Court decisions applying the law of nations necessarily presupposed that the political branches may depart from the law of nations in their respective constitutional powers. Because decisions regarding when and whether to adhere to - or depart from - the law of nations “are rather questions of policy than of law,” the Constitution’s allocation of powers assigned such decisions to the political branches of the federal government. In addition, we offer a separation of powers rationale for why the Court has sometimes limited executive power according to the law of nations while leaving Congress free to depart from such law. On this account, judicial enforcement of the law of nations against the Executive Branch appears to track the Court’s understanding of the Constitution’s allocation of powers between Congress and the President. Because the Constitution assigns all foreign affairs powers to Congress and the President, however, the Court has never suggested that courts could enforce the law of nations to constrain the collective constitutional power of the political branches