Posse Comitatus Act Violation at G20 Demonstrations, September 24, 2009

The Posse Comitatus Act is a United States federal law (18 U.S.C. § 1385) passed on June 18, 1878, after the end of Reconstruction, with the intention (in concert with the Insurrection Act of 1807) of substantially limiting the powers of the federal government to use the military for law enforcement. The Act prohibits most members of the federal uniformed services (today the Army, Air Force, and State National Guard forces when such are called into federal service) from exercising nominally state law enforcement, police, or peace officer powers that maintain "law and order" on non-federal property (states and their counties and municipal divisions) within the United States.
The statute generally prohibits federal military personnel and units of the National Guard under federal authority from acting in a law enforcement capacity within the United States, except where expressly authorized by the Constitution or Congress. The Coast Guard is exempt from the Act during peacetime.