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, (1) the Court followed the law of nations not because it believed such law qualified as "the supreme Law of the Land," (2) but 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. (3) Had the judiciary taken it upon itself to depart from well-established principles of the law of nations, it would have usurped the authority of Congress and the President to decide whether, when, and how to depart from such law in the conduct of foreign relations. 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 also rested on an important, but largely overlooked, predicate--that is, "that the political branches were free to make law in derogation of the law of nations, and that such law would bind courts as the supreme law of the land." (4) Because decisions regarding when and whether to adhere to--or depart from--the law of nations "are rather questions of policy than of law," (5) the Constitution's allocation of powers assigned such decisions to the political branches of the federal government. (6) In other words, courts generally followed the law of nations in the early republic not as a form of supreme federal law in itself, but as a means of preserving the political branches' exclusive constitutional prerogatives to decide whether, when, and how to depart from such law.
Scholars have long debated whether courts should uphold actions by the political branches that depart from the law of nations. In addition, some distinguish between congressional and executive action in this regard. Most scholars agree that courts must uphold acts of Congress that depart from the law of nations. The most prominent advocate of this position is Louis Henkin. "In principle," he claimed, "every state has the power--I do not say the right--to violate international law and obligation and to suffer the consequences." (7) Indeed, in his view, the Constitution recognizes a "national prerogative to violate international law." (8) "If Congress enacts legislation that is inconsistent with, and causes the United States to violate, an established principle of customary law, the Executive and the courts are obliged to give effect to the act of Congress." (9) In other words, "Congress, in legislating under its constitutional powers, can enact law inconsistent with an international agreement or other international obligation of the United States, thereby causing the United States to be in violation of that agreement or obligation." (10)
A few scholars, however, have asserted that courts should enforce established rules of customary international law to restrict congressional action. Jordan Paust, for instance, has argued that "well into the 20th Century no one expected that the President or Congress could even authorize a violation of customary international law and nothing in the text or structure of the Constitution permits such a result." (11) Jules Lobel has claimed that "fundamental" rules of customary international law are judicially enforceable against acts of Congress (12): "those rules of international law that reflect the fundamental norms of contemporary society bind Congress ... domestically as well as internationally." (13)
Scholars have also debated whether customary international law limits the powers of the President. Several scholars have concluded that it does not, albeit for somewhat different reasons. Some have argued that customary international law is not judicially enforceable against the President because it does not have the status of federal law. …