The statute generally prohibits federal military personnel and units of the United States National Guard under federal authority from acting in a law enforcement capacity within the United States, except where expressly authorized by the Constitution or Congress. The Coast Guard is exempt from the Posse Comitatus Act.
The Posse Comitatus Act and the Insurrection Act substantially limit the powers of the federal government to use the military for law enforcement.
In return for Southern acquiescence in conceding to Rutherford B.Hayes, Republicans withdrew federal troops from Southern states of the former Confederacy, ending Reconstruction. Known as the Compromise of 1877, this deal of political expediency removed federal troops, and as result all federal protection for Southern ex-slaves. With the end of Reconstruction, Southern states were free to enact laws barring African-Americans from serving in government and eventually from voting (e.g., poll taxes and grandfather clauses) Segregated and disenfranchised, African-Americans in the South became second-class citizens or in many instances, de facto slaves to the political and economic order of the post-bellum "Jim Crow" South.
To this day, the US Constitution places primary responsibility for the holding, the conduct, and the promulgation of the rules for elections to federal office to the several states of the Union. The authority of a state's deputized peace officers to maintain the peace, for the orderly conduct of elections, and the prevention of unlawful activity to prevent the exercise of the voting franchise, is a state responsibility; in view of the states' primary job to exercise the police power, the power to maintain law and order.
It was widely recognized that in the former Confederate States during the local, state, and federal elections of 1874 and 1876, the state governments and their county and local municipalities had deliberately elected not to exercise their police powers to maintain law and order during the holding of elections in those years. As a result, many acts of violence and the deliberate suppression of the vote of some political and racial groups resulted in the election of state legislators and U.S. congressmen who would have not been elected otherwise.
When those U.S. representatives—and the senators selected by the newly-formed legislatures of the former Confederate States—reached Washington, they set as their first priority the creation of a statute prohibiting a future president or congress from directing by military order or federal legislation the re-imposition of federal troops who would again exercise such police powers as were necessary to maintain law and order during elections.
The original act referred only to the United States Army. The Air Force was added in 1956, and the Navy and the Marine Corps have been included by a regulation of the Department of Defense. The United States Coast Guard, when acting in its peacetime capacity (originally as part of the Department of Transportation, now within the Department of Homeland Security), is not included in the act. However, if in wartime, a portion of the United States Coast Guard were subsumed within the Department of the Navy, as it was during World War II, that portion, under the act, would lose its federal police power authority and responsibility over the federal law enforcement duties of its civilian mission. This law is often mentioned when it appears that the Department of Defense is interfering in domestic disturbances.
The whole text of the relevant legislation is as follows:
Also notable is the following provision within Title 10 of the United States Code (which concerns generally the organization and regulation of the armed forces and Department of Defense):
Provisions of the Act, including sections 841, 846, 1079, and 1222, purport to impose requirements that could inhibit the President's ability to carry out his constitutional obligations to take care that the laws be faithfully executed, to protect national security, to supervise the executive branch, and to execute his authority as Commander in Chief. The executive branch shall construe such provisions in a manner consistent with the constitutional authority of the President.
GEORGE W. BUSH
THE WHITE HOUSE,
January 28, 2008.
Although it is a military force, the United States Coast Guard, which operates under the Department of Homeland Security, is not covered by the Posse Comitatus Act. The Coast Guard enforces U.S. laws, even when operating as a service for the Navy.
In December 1981 additional laws were enacted clarifying permissible military assistance to civilian law enforcement agencies and the Coast Guard especially in combating drug smuggling into the United States. Posse Comitatus clarifications emphasize supportive and technical assistance (e.g., use of facilities, vessels, aircraft, intelligence, tech aid, surveillance) while generally prohibiting direct participation of Department of Defense personnel in law enforcement (e.g., search, seizure, and arrests). For example, a US Navy vessel may be used to track, follow and stop a vessel suspected of drug smuggling, but a Coast Guard Law Enforcement Detachments (LEDETS) aboard the Navy vessel would perform the actual boarding and, if needed, arrest the crew.
These changes were repealed in their entirety in 2008, except for the provisions of the Presidential signing statement which retained any powers, the repeal of which, the president may determine to be unconstitutional.
On September 30, 2008 the US Army announced that the 3rd Infantry Division’s 1st Brigade Combat Team (BCT) will be under the day-to-day control of US Army North, the Army service component of Northern Command (NORTHCOM), as an on-call federal response force for natural or manmade emergencies and disasters, including terrorist attacks.
This marks the first time an active US Army unit will be given a dedicated assignment to NORTHCOM, where it is stated they may be "called upon to help with civil unrest and crowd control or to deal with potentially horrific scenarios such as massive poisoning and chaos in response to a chemical, biological, radiological, nuclear or high-yield explosive (CBRNE) attack." These soldiers will also learn how to use non-lethal weapons designed to "subdue unruly or dangerous individuals" without killing them, and also includes equipment to stand up a hasty road block; spike strips for slowing, stopping or controlling traffic; shields and batons; and, beanbag bullets.
This also demonstrates the possibility of utilizing federal troops for missions within the United States, essentially eliminating any recognition of Posse Comitatus.
These changes were repealed in their entirety in 2008, except for the provisions of the Presidential signing statement which retained any powers, the repeal of which, the president may determine to be unconstitutional.