Posse Comitatus Act
"The term 'posse comitatus' means the 'force of the county.' Its doctrine dates back to English common law, in which a county sheriff could raise a posse comitatus to repress a civil disturbance or for other purposes. The Posse Comitatus Act of 1878 states (as amended):
"'Whoever, except in cases and under circumstances expressly authorized by the Constitution or Act of Congress, willfully uses any part of the Army or the Air Force as a posse comitatus or otherwise to execute the laws shall be fined under this title or imprisoned not more than two years, or both.'
"Over the past century and a half, the wording of the Posse Comitatus Act has remained essentially unchanged, but its use has become engrained in U.S. law and culture. Though there have been several incidents when the Act has been severely tested, this basic concept of civilian control – absent in too many other countries – has helped guarantee freedom from military oppression."
Withers, George; Santos, Lucila; and Isacson, Adam, "Preach What You Practice: The Separation of Military and Police Roles in the Americas," Washington Office on Latin America (Washington, DC: November 2010), p. 3.