“jus gentium,” (Latin: “law of nations”), in legal theory, that law which natural reason establishes for all men, as distinguished from jus civile, or the civil law peculiar to one state or people. Roman lawyers and magistrates originally devised jus gentium as a system of equity applying to cases between foreigners and Roman citizens. The concept originated in the Romans’ assumption that any rule of law common to all nations must be fundamentally valid and just. They broadened the concept to refer to any rule that instinctively commended itself to their sense of justice.—Encyclopedia Britannica
The U. S. Constitution, in Article I, authorized Congress to define and punish “Offences against the Law of Nations.
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