http://www.law.georgetown.edu/internationalhrcolloquium/documents/RamseyPaper.doc.
II. The Law of Nations as Supreme Law
The Constitution’s Article VI declared that the Constitution, treaties and laws of the United States made up the “supreme Law of the Land.”
This Part instead asks whether and how the Vattel/Blackstone “law of nations” became part of the “supreme Law” defined by Article VI.”
I. The Framers and the Law of Nations
A. The Framers’ Commitment to the Law of Nations
To begin with uncontroversial background, the constitutional generation in America recognized a set of international rights and duties they called the “law of nations.”
The Constitution’s text directly acknowledged this law: Article I, Section 8, gave Congress power to “define and punish … offenses against the law of nations.”
While the Constitution did not define the phrase, it invoked the Enlightenment conception of a system of rights and duties arising outside U.S. domestic law, from the nature of the international system, which was binding (at least in some senses) upon the United States and its citizens.”
“A stronger statement may be from The Nereide (1815), in which Chief Justice Marshall, writing for the Court, said that the Court was bound by the law of nations unless an act of the legislature directed otherwise.”
link above