Posse Comitatus Act

The Posse Comitatus Act is a United States federal law (, original at signed into law on June 18, 1878, by President Rutherford B. Hayes) which makes it a felony for any person to use the federal military to enforce federal law unless authorized to do so by law. Congress passed the act as an amendment to an army appropriations bill following the end of Reconstruction and updated it in 1956, 1981 and 2021.

The act originally applied only to the United States Army, but a subsequent amendment in 1956 expanded its scope to the United States Air Force. In 2021, the National Defense Authorization Act for Fiscal Year 2022 further expanded the scope of the act to cover the United States Navy, Marine Corps, and Space Force. The act does not prevent the Army National Guard or the Air National Guard under state authority from acting in a law enforcement capacity within its home state or in an adjacent state if invited by that state's governor. The United States Coast Guard (under the Department of Homeland Security) is not covered by the act either, primarily because, although it is an armed service, it also has a maritime law enforcement mission.

The title of the act comes from the legal concept of posse comitatus, the common law authority under which a state official can conscript any able-bodied person or group to assist in keeping the peace and enforce the states laws.

History

The act, section 15 of the appropriations bill for the Army for 1879 (found at ) was a response to, and subsequent prohibition of, the military occupation of the former Confederate States by the United States Army during the twelve years of Reconstruction (1865–1877) following the American Civil War (1861–1865).

The U.S. Constitution places primary responsibility for the holding of elections in the hands of the individual states. The maintenance of peace, conduct of orderly elections, and prosecution of unlawful actions, are all state responsibilities according to any state's role of exercising police power and maintaining law and order, whether part of a wider federation or as a unitary state. However, in the former Confederate States, many paramilitary groups sought to suppress, often through intimidation and violence, African-American political power and return the South to rule by the predominantly white Democratic Party. Although African Americans were initially supported by the federal government, as Reconstruction went on, that support waned. Following the bitterly disputed 1876 U.S. presidential election and Compromise of 1877, Congressmen and Senators from the former Confederate States returned to Washington and prioritized prohibiting the federal government from reimposing control over their states. After President Hayes used federal troops to end the Great Railroad Strike of 1877, there was sufficient bipartisan support to pass what became the Posse Comitatus Act. The Space Force, established in 2020, was also included in the act in 2021. The United States Coast Guard is not included in the act even though it is part of the six armed services as it is explicitly given federal law enforcement authority on maritime law. The modern Coast Guard did not exist at the time the act became law in 1878. Its predecessor, the United States Revenue Cutter Service, was primarily a customs enforcement agency and part of the United States Department of the Treasury. In 1915, when the Revenue Cutter Service and the United States Lifesaving Service were amalgamated to form the Coast Guard, the service was both made a military branch and given federal law enforcement authority.

In the mid-20th century, the administration of President Dwight D. Eisenhower used an exception to the Posse Comitatus Act, derived from the Enforcement Acts, to send federal troops into Little Rock, Arkansas, during the 1957 school desegregation crisis. The Arkansas governor had opposed desegregation after the United States Supreme Court ruled in 1954 in Brown v. Board of Education that segregated public schools were unconstitutional. The Enforcement Acts, among other powers, allowed the president to call up military forces when state authorities were either unable or unwilling to suppress violence that was in opposition to the citizens' constitutional rights.

In the summer of 2020, the George Floyd protests in Washington, D.C., generated controversy when National Guard troops were called in to suppress protests without President Donald Trump's invoking the Insurrection Act. One set of troops, the District of Columbia National Guard, has historically operated as the equivalent of a state militia (under Title 32 of the United States Code) not subject to Posse Comitatus Act restrictions, even though it is a federal entity under the command of the President and the Secretary of the Army. National Guard troops from cooperative states were also called in at the request of federal agencies, some of whom were deputized as police. Attorney General William Barr cited ,

In 2020, U.S. representative Adam Schiff introduced an amendment to the act to expand its coverage to include the U.S. Navy, Marine Corps, and Space Force. This amendment was eventually included in the 2022 National Defense Authorization Act.

The 2025 lawsuit Newsom v. Trump raised constitutional and statutory challenges under the Posse Comitatus Act in response to the domestic deployment of U.S. Marines and National Guard troops in California. On September 2, 2025, federal judge Charles Breyer ruled that the federal deployment of the National Guard in California violated the Posse Comitatus Act and issued an injunction against it.

Legislation

The original provision of 1878 was enacted as Section 15 of chapter 263, of the acts of the 2nd session of the 45th Congress.

The text of the relevant legislation following its amendment in 2021 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):

2006–2007 suspension

In 2006, Congress modified the Insurrection Act as part of the 2007 Defense Authorization Bill (repealed as of 2008). On September 26, 2006, President George W. Bush urged Congress to consider revising federal laws so that U.S. armed forces could restore public order and enforce laws in the aftermath of a natural disaster, terrorist attack or incident, or other condition. These changes were included in the John Warner National Defense Authorization Act for Fiscal Year 2007 (), which was signed into law on October 17, 2006.

Section 1076 is titled "Use of the Armed Forces in major public emergencies". It provided that:

In 2008, these changes in the Insurrection Act of 1807 were repealed in their entirety, reverting to the previous wording of the Insurrection Act. It was initially written to limit presidential power as much as possible in the event of insurrection, rebellion, or lawlessness.

Exceptions, exclusions, and limitations

There are several situations in which the act does not apply. These include:

General criminal and criminal counterintelligence investigations, when there is a nexus to a particular military branch, conducted by civilian agents of certain independent military investigative agencies, such as: Air Force Office of Special Investigations (OSI), Naval Criminal Investigative Service (NCIS), United States Army Criminal Investigation Division (CID), and United States Army Counterintelligence Command (ACIC). The Posse Comitatus Act (PCA) specifically states, "except in cases and under circumstances expressly authorized by the Constitution or Act of Congress" with the following Acts of Congress providing specific statutory law enforcement authorities to include:
Under , civilian Special Agents of ACIC and CID can execute and serve arrest and search warrants and likewise pertains to investigative activities associated with those authorities; however, unlike CID which is under a civilian director, ACIC agents remain under the control of a military commander.
Under , civilian Special Agents of NCIS can execute and serve arrest and search warrants and likewise pertains to investigative activities associated with those authorities.
Under , civilian Special Agents of OSI can execute and serve arrest and search warrants and likewise pertains to investigative activities associated with those authorities.

Title 32 establishes and affirms that National Guard units are controlled by their respective state governors by default. While the Guard remains under state command, the Posse Comitatus Act does not apply. However, under , the President may place a state’s National Guard under federal command if (1) the United States is invaded, (2) there is a rebellion or danger of rebellion against federal authority, or (3) the President is unable to enforce federal law with existing forces. Once federalized under Title 10, National Guard personnel are treated as active-duty military and become subject to the Posse Comitatus Act. Separately, under (f), the President or Secretary of Defense can request that a state governor issue orders for members of that state’s National Guard to perform duties, including law enforcement functions. However, in this case, the National Guard remains under the control of the governor and the governor retains legal authority to refuse the request. Because the Guard remains under state command in Title 32 status, the Posse Comitatus Act does not apply.

Federal troops used in accordance to the Insurrection Act, which has been invoked 23 times, .

Under , the Attorney General may request that the Secretary of Defense provide emergency assistance if domestic law enforcement is inadequate to address specific types of threats involving the release of nuclear materials, such as potential use of a nuclear or radiological weapon. Such assistance may be by any personnel under the authority of the Department of Defense, provided such assistance does not adversely affect U.S. military preparedness. The only exemption is the deployment of nuclear materials on the part of the United States Armed Forces.

Under , the Attorney General may request assistance from the military in enforcement of laws regarding biological, chemical, or other weapons of mass destruction (, , and ).

Under , , and , the Attorney General may request that any other federal, state, or local agency, including the military, assist in the enforcement of laws protecting foreign diplomats and their families.

Under , the military may assist in responding to terrorist bombings, particularly in terms of explosive ordnance disposal.

Under , the military may provide surveillance, intelligence gathering, observation, and equipment for domestic law enforcement on operations such as drug interdiction and counter-terrorism missions. For example, Delta Force soldiers from Fort Bragg were deployed upon request by the Federal Bureau of Investigation to serve as sniper/observer teams, run communications, provide medical support, gather intelligence, and conduct assistance in explosive breaching during the 1987 Atlanta prison riots.

Exclusion applicable to U.S. Coast Guard

Although it is an armed service, the U.S. Coast Guard, which operates under the United States Department of Homeland Security during peacetime, is not restricted by the Posse Comitatus Act and has explicit authority to enforce federal law. This is true even when the Coast Guard operates as a service within the United States Navy during wartime.

References

External links

Use of Army and Air Force as Posse Comitatus
"The Posse Comitatus Act: A Principle in Need of Renewal", Washington University Law Quarterly, Vol. 75, No. 2
Should the Posse Comitatus Act be Changed to Effectively Support Local Law Enforcement?—U.S. Army War College
John Warner National Defense Authorization Act for Fiscal Year 2007
Bill Text 112th Congress (2011–2012) S.1867.ES
Federalist Paper #29 Concerning the Militia, Alexander Hamilton, pub. 10 January 1788