Article IV, Section 1:
Full Faith and Credit shall be given in each State to the public Acts, Records, and judicial Proceedings of every other State. And the Congress may by general Laws prescribe the Manner in which such Acts, Records and Proceedings shall be proved, and the Effect thereof.
Doubtless Congress, by virtue of its powers in the field of foreign relations, might also lay down a mandatory rule regarding recognition of foreign judgments in every court of the United States. At present the duty to recognize judgments even in national courts rests only on comity and is qualified in the judgment of the Supreme Court, by a strict rule of parity. 1