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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 members of the Army 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 prohibits Army and Air Force 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 Navy and Marine Corps are prohibited by a Department of Defense directive, not by the Act itself.[1][2] The Coast Guard, under the Department of Homeland Security, is exempt from the Act.

http://en.wikipedia.org/wiki/Posse_Comitatus_Act

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