Article I, Section 8 of the U.S. Constitution contains the obscure but significant “Offenses Clause“, which empowers Congress to “define and punish … Offenses against the Law of Nations.” The law of nations was the 18th century term for what we now call international law.
As the time, these “offenses” would have included “attacks on foreign nations, their citizens, or shipping;” failing to honor “the flag of truce, peace treaties, and boundary treaties” (including unauthorized entry across national borders); and mistreating prisoners of war. The law of nations also obliged states to prosecute pirates, protect wrecked ships and their crew (regardless of their nationality); and protect foreign dignitaries and merchants in their territory.
Thus, the Framers clearly sought to convey to the world that the U.S. would be a responsible actor among the global community, enshrining in its highest legal instrument a commitment to safeguarding foreign nationals, property, and interests, even if it means ostensibly prosecuting American perpetrators.
Some jurists have argued that this provision, in theory, permits Congress to criminalize private conduct in the U.S. that violates international law.