Posse comitatus is a Latin term that literally means “force of the county.” The principle of posse comitatus is the legal concept that prevents members of the military from acting as a police force in the United States. Established in 1878 via the Posse Comitatus Act, this principle prohibits the use of federal military personnel or personnel from the National Guard, when in a state active duty status, to participate in law enforcement activities within the United States.
In other words, posse comitatus was designed to provide clear separation between civil law enforcement and the military. The need for separation was especially evident in the American South during Reconstruction. Civil law enforcement should never become an arm of the military— a concept that goes back to English common law and the time of the Magna Carta.
In modern times, there have been occasions when federal troops were used to aid civil law enforcement. The most common occasions in which on-duty military personnel were used to provide assistance to civil law enforcement have been in disaster relief situations. Examples of this include the Hurricane Katrina disaster in 2005 and the Los Angeles riots in 1992.
The prohibition on posse comitatus may be overridden by specific congressional statutes, but only in the event of a natural disaster, civil unrest, or other pressing public need. Otherwise, federal military personnel or personnel of the National Guard, when in a state active-duty status, are typically not authorized to conduct law enforcement activities.