Article I, Section 8, Clause 9: (Congress shall have power...) To define and punish Piracies and Felonies committed on the high Seas,
and Offences against the Law of Nations;
The Law of Nations is defined by Blackstone's Commentaries:
Briefly, the Law of Nations at the point of ratification in 1788 included the following general elements, taken from Blackstone's Commentaries, and prosecution of those who might violate them:
(2) Honoring of the flag of truce, peace treaties, and boundary treaties. No entry across national borders without permission of national authorities.
Repeating: "The Law of Nations" is referenced in the Constitution as among the POWERS OF CONGRESS. The LAW OF NATIONS includes NO ENTRY ACROSS NATIONAL BORDERS WITHOUT PERMISSION OF NATIONAL AUTHORITIES.
http://www.constitution.org/cmt/law_of_nations.htm
HT: JeffB