Posse comitatus (common law)
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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 posse comitatūs, "to have the right to an armed retinue", 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.
United Kingdom 
- English Civil War
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.
- In law
The powers of sheriffs for posse comitatus were codified by section 8 of the Sheriffs Act 1887, the first subsection of which stated that:
Every person in a county shall be ready and apparelled at the command of the sheriff and at the cry of the country to arrest a felon whether within a franchise or without, and in default shall on conviction be liable to a fine, and if default be found in the lord of the franchise he shall forfeit the franchise to the Queen, and if in the bailiff he shall be liable besides the fine to imprisonment for not more than one year, or if he have not whereof to pay the fine, than two years.
This permitted the (high) sheriff of each county to call every citizen to his assistance to catch a person who had committed a felony—that is, a serious crime. It provided for fines for those who did not comply. The provisions for posse comitatus were repealed by the Criminal Law Act 1967. The second subsection provided for the sheriff to take 'the power of the county' if he faced resistance whilst executing a writ, and provided for the arrest of resisters. This subsection is still in force.
United States 
Every law enforcement officer is bound to execute the penal warrants given to him to execute. He may summon to his assistance, either in writing or orally, any of the citizens of the neighborhood or county to assist in the execution of such warrants. The acts of the citizens formed as a posse by such officer shall be subject to the same protection and consequences as official acts.
Resort to the posse comitatus figures often in the plots of Western movies, where the body of men recruited is frequently referred to as a posse. Based on this usage, the word posse has come to be used colloquially to refer to various teams, cliques, or gangs, often in pursuit of a crime suspect (on horseback in the westerns), sometimes without legal authority. In a number of states, especially in the Western United States, sheriffs and other law enforcement agencies have called their civilian auxiliary groups "posses." The Lattimer Massacre of 1897 illustrated the danger of such groups, and thus ended their use in situations of civil unrest.
In the United States, a federal statute known as the Posse Comitatus Act forbids the use of the United States Army, and through it, its offspring, the United States Air Force as a posse comitatus or for law enforcement purposes. A directive from the Secretary of Defense prohibits the use of the United States Navy and United States Marine Corps for law enforcement. No such limitation exists on the United States Coast Guard, which can be used for all law enforcement purposes (for example, Coast Guardsmen were used as temporary Air Marshals for many months after the 9/11 attacks) except when, as during WWII, a part of the Coast Guard is placed under the command of the Navy. This part would then fall under the regulations governing the Navy in this matter, rather than those concerning the Coast Guard. The limitation also does not apply to the National Guard when activated by a state's governor and operating in accordance with Title 32 of the U.S. Code (for example, National Guardsmen were used extensively by state governors during Hurricane Katrina response actions). Conversely, the limitation would apply to the National Guard when activated by the President and operating in accordance with Title 10 of the U.S. Code.
Members of the posse that killed Ned Christie posing with his corpse in November 1892.
Deputy William Banks (left) and Deputy Marshal Isaac S. Prater (right) were among the posse that killed William Blake (center) in 1895.
Possemen and a horse thief at Judge Roy Bean's saloon in c.1900. The thief is sitting on a horse below the "Ice Beer" sign, with his hands behind his back.
A posse that was assembled to fight Butch Cassidy's Wild Bunch in 1900.
A posse hanging a horse thief in Oregon, c.1900.
Striking miners confronting an American posse during the Cananea Riot in 1906.
A posse rounding up strikers during the Bisbee Deportation in 1917.
See also 
|Wikisource has the text of the 1911 Encyclopædia Britannica article Posse comitatus.|
- "Ruby, Arizona – A Ghost Town Filled With Mining and Murder". Legends of America. p. 3.
- Section 8 of Sheriffs Act 1887
- OED, s.v. "posse n. 2, posse comitatus.
- "All the Posse of Hell, cannot violently eject me." T. Fuller, Good Thoughts in Bad Times (1645) I. xv. 39. "A whole posse of the young lady's kindred—brothers, cousins and uncles—stood ready at the street door to usher me upstairs." W. Beckford Portuguese Jrnl. 10 June 1787, p. 72. (cited after OED).
- "I can lick the whole posser-commertatus of yer. Come on, yer cowards!" Harper's Magazine July 1862, 184/1 (cited after OED).
- Schedule 3, Part III, Criminal Law Act 1967
- section 8, Sheriffs Act 1887 (as passed)
- section 8, Sheriffs Act 1887 (as amended)
- U. S. Code Title 10 and Title 32