Posse Comitatus Act

The Posse Comitatus Act is the United States federal law (18 U.S.C. § 1385, original at 20 Stat. 152) that was passed on June 18, 1878, after the end of Reconstruction. Its intent (in concert with the Insurrection Act of 1807) was to limit the powers of local governments and law enforcement agencies in using federal military personnel to enforce the laws of the land. Contrary to popular belief, the Act does not prohibit members of the Army from exercising state law enforcement, police, or peace officer powers that maintain "law and order"; it simply requires that any authority to do so must exist with the United States Constitution or Act of Congress. In this way, most use of the Army and the Air Force at the direction of the President does not offend the statute, even though it may be problematic for political reasons.

The statute only addresses the US Army and, since 1956, the US Air Force. It does not refer to, and thus does not restrict or apply to, the 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 (in its federal capacity, the National Guard forms part of the Army or Air Force of the United States). The Navy and Marine Corps are prohibited by a Department of Defense directive (self-regulation), but not by the Act itself. Although it is a military force, the U.S. Coast Guard, which now operates under the Department of Homeland Security, is also not covered by the Posse Comitatus Act, primarily because the Coast Guard has both a maritime law enforcement mission and a federal regulatory agency mission.