The Posse Comitatus Act is the United States federal law (18 U.S.C. § 1385) 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 from using federal military personnel to enforce the laws of the land.
http://en.wikipedia.org/wiki/Posse_Comitatus_Act