The Posse Comitatus Act is a United States federal law signed on June 18, 1878 by President Rutherford B. Hayes. The purpose of the act – in concert with the Insurrection Act of 1807 – is to limit the powers of the federal government in using federal military personnel to enforce domestic policies within the United States. It was passed as an amendment to an army appropriation bill following the end of Reconstruction, and was subsequently updated in 1956 and 1981.
The Act only specifically applies to the U.S. Army and, as amended in 1956, the U.S. Air Force. While the Act does not explicitly mention the U.S. Navy and the U.S. Marine Corps, due to their being naval services, the Navy has prescribed regulations that are generally construed to give the Act force with respect to those services as well. The Act does not apply to the Army and 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 U.S. Coast Guard, which operates under the Department of Homeland Security, is not covered by the Posse Comitatus Act either, primarily because although the Coast Guard is an armed service, it also has both a maritime law enforcement mission and a federal regulatory agency mission.