Posse Comitatus Act, 1878, U.S. federal law that makes it a crime to use the military as a domestic police force in the United States under most circumstances. The law was designed to end the use of federal troops to supervise elections in the post–Civil War South. The posse comitatus (from which the term posse derives) is the power or force of the county, and refers to citizens above the age of 15, who may be summoned by a sheriff to enforce the law. The act specifically prohibited the use of the U.S. army as a posse comitatus; the prohibition was later extended by legislation to the air force and by government directive to the marine corps and navy. The restriction does not apply to the coast guard during peacetime or the national guard when it is under state authority. There are legal exceptions to the law, particularly in aspects of drug law enforcement, in emergency situations, and in cases of rebellion.