Posse Comitatus Act

The Posse Comitatus Act is a United States federal law (18 U.S.C. § 1385, original at 20 Stat. 152 signed into law on June 18, 1878, by President Rutherford B. Hayes) which makes it a felony for any person to use the federal military to enforce federal law unless authorized to do so by law. Congress passed the act as an amendment to an army appropriations bill following the end of Reconstruction and updated it in 1956, 1981 and 2021.

The act originally applied only to the United States Army, but a subsequent amendment in 1956 expanded its scope to the United States Air Force. In 2021, the National Defense Authorization Act for Fiscal Year 2022 further expanded the scope of the act to cover the United States Navy, Marine Corps, and Space Force. The act does not prevent the Army National Guard or the Air National Guard under state authority from acting in a law enforcement capacity within its home state or in an adjacent state if invited by that state's governor. The United States Coast Guard (under the Department of Homeland Security) is not covered by the act either, primarily because, although it is an armed service, it also has a maritime law enforcement mission.

The title of the act comes from the legal concept of posse comitatus, the common law authority under which a state official can conscript any able-bodied person or group to assist in keeping the peace and enforce the states laws.