The Posse Comitatus Act is a United States federal law (18 U.S.C. § 1385, original at 20 Stat. 152) signed on June 18, 1878, by President Rutherford B. Hayes which limits the powers of the federal government in the use of federal military personnel to enforce domestic policies within the United States. The Act was passed as an amendment to an army appropriation bill following the end of Reconstruction and was updated in 1956 and 1981.
The title of the Act comes from the legal concept of posse comitatus, the authority under which a county sheriff, or other law officer, can conscript any able-bodied person to assist in keeping the peace.
The posse comitatus (from the Latin for "power of the county"), in common law, is a group of people mobilized by the conservator of peace – typically a sheriff – to suppress lawlessness or defend the county. The posse comitatus originated in ninth century England simultaneous with the creation of the office of sheriff. Though generally obsolete throughout the world, it remains theoretically, and sometimes practically, part of the United States legal system.