Article I, Section 8, Clause 10:
[The Congress shall have Power . . .] To define and punish Piracies and Felonies committed on the high Seas, and Offences against the Law of Nations; . . .
The fact that the Constitutional Convention considered it necessary to give Congress authority to define offenses against the law of nations does not mean that in every case Congress must undertake to codify that law or mark its precise boundaries before prescribing punishments for infractions thereof. An act punishing
the crime of piracy, as defined by the law of nations punishing the was held to be an appropriate exercise of the constitutional authority to
define and punish the offense, since it adopted by reference the sufficiently precise definition of International Law. 1 Similarly, in Ex parte Quirin, 2 the Court found that by the reference in the Fifteenth Article of War to
offenders or offenses that . . . by the law of war may be triable by such military commissions . . ., Congress had
exercised its authority to define and punish offenses against the law of nations by sanctioning, within constitutional limitations, the jurisdiction of military commissions to try persons for offenses which, according to the rules and precepts of the law of nations, and more particularly the law of war, are cognizable by such tribunals. 3 Where, conversely, Congress defines with particularity a crime which is
an offense against the law of nations, the law is valid, even if it contains no recital disclosing that it was enacted pursuant to this clause. Thus, the duty which the law of nations casts upon every government to prevent a wrong being done within its own dominion to another nation with which it is at peace, or to the people thereof, was found to furnish a sufficient justification for the punishment of the counterfeiting within the United States, of notes, bonds, and other securities of foreign governments. 4