Posse comitatus is a concept in common law which states that law enforcement officials may conscript able-bodied individuals, classically limited to men, for the purpose of specific law enforcement tasks. While many nations technically permit posse comitatus to occur, it is extremely rare to see law enforcement conscripting civilians, despite the depictions in Western films and novels. The term “posse comitatus” is also used in reference to the Posse Comitatus Act of the United States, which explicitly forbids the use of American military forces for law enforcement activities, with the exception of the Coast Guard.
This common law concept is centuries old, dating back to a time before law enforcement was viewed as a highly specialized task which required unique training. Sheriffs could muster up able-bodied men to keep the peace during periods of civil unrest, to seek out criminals, and to provide additional security as needed. The posse comitatus concept was sometimes invoked by opposing sides, as seen during the English Civil War, in which both sides had the ability to conscript civilians.
Law enforcement officers are more likely to commandeer vehicles, another grossly-exaggerated law enforcement power, than they are to conscript civilians, due to safety issues. Civilians are not trained to participate in law enforcement tasks, and they could endanger themselves or others if they were impressed into a posse. In emergency situations where law enforcement officers do start deputizing civilians, they will likely seek out retired law enforcement and people with military experience.
Under the Posse Comitatus Act, members of the American military cannot be pressed into service for law enforcement tasks. One of the key ideas behind the act was that its architects did not want to see the American military working domestically, and the act was also designed to limit the powers of the federal government. In point of fact, members of the National Guard and Coast Guard are used for law enforcement tasks in the United States and abroad, leading some people to suggest that this Act should probably be revised to reflect the shifting nature of the United States military.
Thanks to the frequent use of the posse comitatus as a plot device in Westerns, many people refer to a group of individuals conscripted by a law enforcement officials as a “posse.” The term “posse” has also come to be used more generally used to refer to a group rallied around a single individual or cause.