In an article in the Yale Law Journal from June 2009, titled: The constitutional power to interpret international law, there is an interesting section dealing with the types of international law and their relationship to the Constitution of the United States. On this web site, we frequently refer to “The Law of Nations” by the Swiss philosopher and jurist Emmerich de Vattel, and it’s assertion that the indigenous people are those who are born in the country of citizen parents. (Later post constitutional English translations use the phrase “natural born citizen.”) The underpinning concept in his philosophical treatise is “natural law” as stated in the full title to the work: The Law of Nations or Principles of the Law of Nature Applied to the Conduct and Affairs of Nations and Sovereigns.
The article talks about “customary international law,” which is the kind of international that de Vattel writes about. It makes it clear that the constitutional phrase “The Law of Nations” does not refer at all to the work previously cited, nor is de Vattel even mentioned in this article.
The third type of international law, what is (customarily) referred to as customary international law, is the foggiest type of all. It refers to the norms and practices of nations, apart from treaties or other written agreements. Within the regime of international law, it is “law” inferred from “a general and consistent practice of states followed by them from a sense of legal obligation.” It is, in effect, a body of unwritten international “common law” principles. As such, the system of international law regards it as just as binding as treaties or other written conventions. Before so much of international law became treaty-fled in the late nineteenth and twentieth centuries, such customary international law, referred to at the time of the Framing of the Constitution as the Law of Nations, was the dominant form of international law. Indeed, it would not be far wrong to refer to international law, at the time of the Framing of the Constitution, as largely consisting of principles of natural law, applicable to the conduct of nations (and their citizens) toward each other on the international plane. What we today call customary international law was, originally, a body of principles of just, proper, and proportionate conduct–right conduct–deduced from general principles of natural justice.
What is the force of customary international law as a constraint on the United States, as a matter of U.S. constitutional law? The short answer is that customary international “norms,” not embodied in treaties to which the United States is a party, are not part of the Article VI “supreme Law of the Land” of the United States at all. Such norms are not “law” made in accordance with U.S. constitutional processes, as specified in Article I (legislation), Article II (treaties), or Article V (constitutional amendments)–the three processes set forth in the Constitution for the making of the three types of federal law. Accordingly, customary international law is not binding in any form on the President’s conduct of foreign affairs or on the exercise of any of his constitutional powers (including the Commander-in-Chief power to conduct war). The same holds true for Congress and the courts: customary international law is not in any constitutional way a binding constraint on the exercise by Congress of any of its constitutional legislative powers, nor does it validly supply a binding federal legal rule of decision in U.S. courts that ever prevails over other law.
That is not to say that customary international law is utterly irrelevant. To the contrary, such customary norms are a kind of international common law that the United States may choose to follow and apply as a matter of our foreign relations policies or practices (as the President determines), as a predicate and informative source for the exercise of Congress’s enumerated legislative power to “define and punish … Offences against the Law of Nations,” and as a source of common law norms for the exercise of the admiralty jurisdiction of federal courts (in the absence of contrary treaty or statutory law). As I discuss below, the presence of international law norms can furnish the basis for the exercise of U.S. constitutional powers, in the exercise of policymakers’ policy discretion and judgment. Customary international law is properly “part of our law” in the sense that principles of natural international law, customary and well-accepted international practice, and the evolving norms of the international community may inform and justify the exercise of several U.S. government constitutional powers.
But as a matter of U.S. law, such international law never prevails over contrary enacted U.S. law or the otherwise-legitimate exercise of a constitutional power possessed by any of the branches of the U.S. government. Customary international law is simply not, and cannot be, binding on the United States as a matter of U.S. constitutional law, because it is not part of the binding “law” identified in Article VI and is not made exclusively by U.S. constitutional actors in accordance with U.S. constitutional processes laid out in Articles I, II, and V. [Emphasis added, internal citations omitted.]