Posse comitatus

Posse comitatus is the common-law or statute law authority of a county sheriff, or other law officer, to conscript any able-bodied man to assist him in keeping the peace or to pursue and arrest a felon, similar to the concept of the "hue and cry." Originally found in English common law, it is generally obsolete; however, it survives in the United States, where it is the law enforcement equivalent of summoning the militia for military purposes.