The specific power given to Congress " To define and punish piracies and felonies committed on the high seas, and offences against the law of nations " (Const. Art. I, § 8, ¶ 10) brings within the scope of federal legislation acts directly affecting the relations of this and foreign governments. The high seas furnish the channels of intercourse with foreign nations, and although they are outside of the jurisdiction of any of the states they are still within the jurisdiction of the United States to this extent, that vessels registered under the laws of the United States, while on the seas, are regarded as parts of the territory of the United States; and criminal acts committed on such vessels are deemed to have been committed within the jurisdiction of the United States. A robbery or forcible depredation on the high seas without lawful authority constitutes piracy (UnitedStates v. Smith), and the offender is subject to punishment, if brought into the United States, as for a crime against the United States, although he may not have been at the time of the commission of the crime a subject of the United States. Pirates are treated as the enemies of all nations, and they are subject to punishment in any jurisdiction into which they may be brought.

Felonies, that is, crimes of a grave nature, committed by persons who are on vessels authorized to sail under the United States flag, are punishable by the laws of the United States, on the theory that the crime is committed within its jurisdiction. Under their admiralty and maritime jurisdiction, the courts of the United States may punish other crimes as defined by Congress, committed on United States vessels; so that it may be said in general that it is within the power of Congress to provide for the punishment of crimes committed on the high seas or on the navigable waters of the United States without regard to whether they are felonies or crimes of lesser degree (United States v. Rodgers).