The Posse Comitatus Act is a federal statute that prohibits the use of the U.S. military in civilian law enforcement, according to Oxford Dictionaries. The notion is based on English law, which called upon citizens to assist the sheriff in pursuing criminals, according to The Free Dictionary.
The actions of a posse comitatus were limited by the Posse Comitatus Act of 1878, which prohibited the use of the U.S. Army to enforce laws unless the Constitution or an act of Congress explicitly authorized such use. This act was amended five times in the 1980s to allow for the use of military resources to combat trafficking in illegal drugs, according to The Free Dictionary. The phrase "posse comitatus" came from the Latin meaning "power of the country," according to Oxford Dictionaries.