Rule of International Law
Now I Am Not A Lawyer, fortunately for me, but this did ring some bells in a recess of my memory.
A few seconds with google brings this up...
John Marshall and International Law
John Marshall became Chief Justice of the Supreme Court in 1801 at the age of forty-six. During his thirty-five year tenure ( 1801-1835), he wrote opinions in approximately eighty cases involving international law. 1 Unquestionably until eight years before his appointment, Marshall's view of the world was largely insular. Only in 1793 did he begin to focus on international problems and to work toward resolving them through the application of international law."
Now I know precedent can be changed etc and so forth - but there does seem to be some wee bit of history on SCOTUS taking account of the laws and customs of nations - possibly up to and including the Treaty of Westphalia...
This of course not factoring in the "treaty" clause of the Constitution, or indeed the US signing the UN Charter (hell, the US wrote the UN Charter - don't complain now!)