The Posse Comitatus Act is a U.S. law enacted in 1878 that limits the powers of the federal government to use military forces for domestic law enforcement. It was established to prevent the military from intervening in civilian affairs, ensuring that local and state authorities maintain control over law enforcement. Under the act, the U.S. Army and U.S. Air Force cannot be used to enforce laws unless explicitly authorized by the Constitution or Congress. However, exceptions exist, such as during national emergencies or when the military is called to assist in disaster relief efforts.