Posse Comitatus Act
"Americans have a tradition, born in England and developed in the early years of our nation, that rebels against military involvement in civilian affairs. It finds its most tangible expression in the nineteenth century Posse Comitatus Act, 18 U.S.C. 1385. The Act forbids use of the Army and Air Force to execute civil law except where expressly authorized."
Doyle, Charles, "The Posse Comitatus Act and Related Matters: The Use of the Military to Execute Civilian Law," Congressional Research Service (Washington, DC: Library of Congress, June 1, 2000), p. 1.