Posse comitatus (common law)

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.

The term derives from the Latin , power or force of the county, in English use from the late 16th century, shortened to posse from the mid 17th century. While the original meaning refers to a group of citizens assembled by the authorities to deal with an emergency (such as suppressing a riot or pursuing felons), the term posse is also used for any force or band, especially with hostile intent, often also figuratively or humorously. In 19th-century usage, posse comitatus also acquires the generalized or figurative meaning.

In 1642, during the early stages of the English Civil War, local forces were employed everywhere by all sides that could. They produced valid written authority, inducing them to assemble. The two most common authorities used were, on the side of the Parliament, its own recent "Militia Ordinance"; or that of the king, the old-fashioned "Commissions of Array". But the Royalist leader in Cornwall, Sir Ralph Hopton, indicted the enemy before the grand jury of the county as disturbers of the peace, and had the posse comitatus called out to expel them.

...
Wikipedia