What is the Posse Comitatus Act?


The Posse Comitatus Act is a United States federal law (18 U.S.C. § 1385, original at 20 Stat. 152) signed on 18 June 1878, by President Rutherford B. Hayes which limits the powers of the federal government in the use of federal military personnel to enforce domestic policies within the United States.

The Act was passed as an amendment to an army appropriation bill following the end of Reconstruction and was updated in 1956 and 1981.

The Act specifically applies only to the United States Army and, as amended in 1956, the United States Air Force. Although the Act does not explicitly mention the United States Navy or the United States Marine Corps, the Department of the Navy has prescribed regulations that are generally construed to give the Act force with respect to those services as well. 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) and United States Space Force (under the Department of the Air Force) are not covered by the Act either, primarily because although both are armed services, they also have maritime and space law enforcement missions respectively.

The title of the Act comes from the legal concept of posse comitatus, the authority under which a county sheriff, or other law officer, can conscript any able-bodied person to assist in keeping the peace.

Brief History

The Act, § 15 of the appropriations bill for the Army for 1879 (found at 20 Stat. 152) 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 US 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, pursuant of any state’s role of exercising police power and maintaining law and order, whether part of a wider federation or a unitary state. However, in the former Confederate States, a number of 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 supported at first by the federal government, as Reconstruction went on, that support waned. Following the bitterly disputed 1876 US presidential election and Compromise of 1877, Congressmen and Senators from the former Confederate States returned to Washington and prioritized prohibiting the federal government from re-imposing 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 original Posse Comitatus Act referred exclusively to the United States Army. The Air Force, established during the 20th century initially as a branch of the Army, was added in 1956. The Navy and Marine Corps are not mentioned in the Act but are subject to the same restrictions by Department of Defence regulation. The United States Coast Guard and United States Space Force are not included in the act even though they are part of the six armed services as both are explicitly given federal law enforcement authority on maritime and space laws respectively. 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 the 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 Trump invoking the Insurrection Act (though he threatened to do so). 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 32 U.S.C. § 502(f)(2)(a), which says National Guard troops may engage in “support of operations or missions undertaken by the member’s unit at the request of the President or Secretary of Defense.” Saying the intent of §502 (titled “Required drills and field exercises”) was to cover training exercises only, Senator Tom Udall and US Representative Jim McGovern described this as a “loophole” to circumvent Posse Comitatus Act restrictions, and introduced legislation to close it.


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

Sec. 15. From and after the passage of this act it shall not be lawful to employ any part of the Army of the United States, as a posse comitatus, or otherwise, for the purpose of executing the laws, except in such cases and under such circumstances as such employment of said force may be expressly authorized by the Constitution or by act of Congress; and no money appropriated by this act shall be used to pay any of the expenses incurred in the employment of any troops in violation of this section and any person wilfully violating the provisions of this section shall be deemed guilty of a misdemeanour and on conviction thereof shall be punished by fine not exceeding ten thousand dollars or imprisonment not exceeding two years or by both such fine and imprisonment.

The text of the relevant legislation is as follows:

18 U.S.C. § 1385. Use of Army and Air Force as posse comitatus Whoever, except in cases and under circumstances expressly authorised by the Constitution or Act of Congress, wilfully uses any part of the Army or the Air Force as a posse comitatus or otherwise to execute the laws shall be fined under this title or imprisoned not more than two years, or both.

Also notable is the following provision within Title 10 of the United States Code (which concerns generally the organisation and regulation of the armed forces and Department of Defence):

10 U.S.C. § 275. Restriction on direct participation by military personnel The Secretary of Defence shall prescribe such regulations as may be necessary to ensure that any activity (including the provision of any equipment or facility or the assignment or detail of any personnel) under this chapter does not include or permit direct participation by a member of the Army, Navy, Air Force, or Marine Corps in a search, seizure, arrest, or other similar activity unless participation in such activity by such member is otherwise authorised by law.

Suspension (2006-2007)

In 2006, Congress modified the Insurrection Act as part of the 2007 Defence Authorisation Bill (repealed as of 2008). On 26 September 2006, President George W. Bush urged Congress to consider revising federal laws so that US 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 Defence Authorisation Act for Fiscal Year 2007 (H.R. 5122), which was signed into law on 17 October 2006.

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

The President may employ the armed forces … to … restore public order and enforce the laws of the United States when, as a result of a natural disaster, epidemic, or other serious public health emergency, terrorist attack or incident, or other condition … the President determines that … domestic violence has occurred to such an extent that the constituted authorities of the State or possession are incapable of maintaining public order … or [to] suppress, in a State, any insurrection, domestic violence, unlawful combination, or conspiracy if such … a condition … so hinders the execution of the laws … that any part or class of its people is deprived of a right, privilege, immunity, or protection named in the Constitution and secured by law … or opposes or obstructs the execution of the laws of the United States or impedes the course of justice under those laws.

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 originally written to limit presidential power as much as possible in the event of insurrection, rebellion, or lawlessness.

Exclusions and Limitations

There are a number of situations in which the Act does not apply. These include:

  • National Guard units, state defence forces, and naval militias while under the authority of the governor of a state.
    • However, when the National Guard is federalised under 10 U.S.C. § 12406, that shifts control from the state governor to the President, making Guard operations subject to the Posse Comitatus Act as well.
  • Federal troops used in accordance to the Insurrection Act, which has been invoked 23 times, as of 2020.
  • Under 18 U.S.C. § 831, the Attorney General may request that the Secretary of Defence provide emergency assistance if domestic law enforcement is inadequate to address certain 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 Defence, provided such assistance does not adversely affect US military preparedness.
    • The only exemption is the deployment of nuclear materials on the part of the United States Armed Forces.
  • 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 US Coast Guard and US Space Force

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

In December 1981, the Military Cooperation with Civilian Law Enforcement Agencies Act was enacted clarifying permissible military assistance to domestic 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, and aircraft, as well as intelligence support, technological aid, and surveillance) while generally prohibiting direct participation of U.S. military 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 Coast Guard Law Enforcement Detachments (LEDETs) embarked aboard the Navy vessel would perform the actual boarding and, if needed, arrest the suspect vessel’s crew.

On 20 December 2019, the National Defence Authorisation Act for Fiscal Year 2020 authorised the creation of the United States Space Force, the sixth US military branch under the United States Department of the Air Force. In February 2020, Air Force Majors Dustin L. Grant and Matthew J. Neil said in their paper The Case for Space: A Legislative Framework for an Independent United States Space Force that the Space Force was established independently to make enforcing US space laws permissible, as such activity would be strictly prohibited under the Air Force branch regarding the Posse Comitatus Act.

Advisory and Support Roles

Federal troops have a long history of domestic roles, including occupying secessionist Southern states during Reconstruction and putting down major urban riots. The Posse Comitatus Act prohibits the use of active duty personnel to “execute the laws”; however, there is disagreement over whether this language may apply to troops used in an advisory, support, disaster response, or other homeland defence role, as opposed to domestic law enforcement.

On 10 March 2009, members of the US Army Military Police Corps from Fort Rucker were deployed to Samson, Alabama, in response to a shooting spree. Samson officials confirmed that the soldiers assisted in traffic control and securing the crime scene. The governor of Alabama did not request military assistance nor did President Obama authorise their deployment. Subsequent investigation found that the Posse Comitatus Act was violated and several military members received “administrative actions”.


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