Overview
The posse comitatus (from the Latin for "power of the county/community/guard"), frequently shortened to posse, is in common law a group of people mobilized by the conservator of peace – typically a reeve, sheriff, chief, or another special/regional designee like an officer of the peace potentially accompanied by or with the direction of a justice or ajudged parajudicial process given the imminence of actual damage – to suppress lawlessness, defend the people, or otherwise protect the place, property, and public welfare (see also ethical law enforcement (police by consent, etc.)). The posse comitatus as an English jurisprudentially defined doctrine dates back to ninth-century England and the campaigns of Alfred the Great (and before in ancient custom and law of locally martialed forces) simultaneous thereafter with the officiation of sheriff nomination to keep the regnant peace (known as "the queen/king's peace"). There must be a lawful reason for a posse, which can never be used for lawlessness.
Etymology
Derived from Latin, posse comitātūs ("force of the county/region") is sometimes shortened to simply posse from the mid-17th century onward to describe the force itself more than the legal principle. 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 and outlawry), the term is also used for any force or band, especially with hostile intent, often also figuratively or humorously. In 19th-century usage, posse comitatus also acquired the generalized or figurative meaning. In classical Latin, posse is a contraction of potesse, an irregular Latin verb meaning "to be able". The unusual genitive in "-ūs" is a feature of the fourth declension. In its earliest days, the posse comitatus was subordinate to the king, country, and local authority.
United Kingdom
English Civil War
In 1642, during the early stages of the English Civil War, local forces were employed everywhere and by all sides. The powers responsible produced valid written authority, inducing the locals to assemble. The two most common authorities used were the Militia Ordinance on the side of the Parliamentarians and 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.
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