Posse Comitatus Act and the US Military on the Border

What the National Guard Can and Cannot Do

National Guard troops getting off C-132 transport in Arizona
Kentucky National Guard Arrives In Arizona. Gary Williams / Getty Images

On April 3, 2018, President Donald Trump proposed that U.S. military troops be deployed along the United States border with Mexico to help control illegal immigration and maintain civil order during construction of the secure, border-length fence recently funded by Congress. The proposal brought questions of its legality under the 1878 Posse Comitatus Act. However, in 2006 and again in 2010, Presidents George W. Bush and Barack Obama took similar actions.

In May 2006, President George W. Bush, in "Operation Jumpstart," ordered up to 6,000 National Guard troops to the states along the Mexican border to support the Border Patrol in controlling illegal immigration and related criminal activities on U.S. soil. On July 19, 2010, President Obama ordered an additional 1,200 Guard troops to the southern border. While this buildup was substantial and controversial, it did not require Obama to suspend the Posse Comitatus Act.

Under Article I of the Constitution, Congress may use the “militia” when necessary “to execute the Laws of the Union, suppress Insurrections and repel Invasions.” It also guarantees that the states will be protected against invasion or attempts to overthrow their “republican form of government,” and, when requested by the state legislature, against “domestic violence.” These constitutional provisions are reflected in the Insurrection Act of 1807 both before and after the passage of the Posse Comitatus Act. The Insurrection Act governs the president's ability to deploy troops within the U.S. to put down lawlessness, insurrection, and rebellion. 

As now expressed by law at 10 U.S. Code § 252, the Insurrection Act is interpreted to mean: “Whenever the President considers that unlawful obstructions, combinations, or assemblages, or rebellion against the authority of the United States, make it impracticable to enforce the laws of the United States in any State by the ordinary course of judicial proceedings, he may call into Federal service such of the militia of any State, and use such of the armed forces, as he considers necessary to enforce those laws or to suppress the rebellion.”

The Posse Comitatus Act limits the Guard troops to acting only in support of the U.S. Border Patrol, and state and local law enforcement officers.

Posse Comitatus and Martial Law

The Posse Comitatus Act of 1878 prohibits the use of U.S. military forces to perform the tasks of civilian law enforcement such as arrest, apprehension, interrogation, and detention unless explicitly authorized by Congress.

The Posse Comitatus Act, signed into law by President Rutherford B. Hayes on June 18, 1878, limits the power of the federal government in the use of federal military personnel to enforce U.S. laws and domestic policies within the borders of the United States. The law was passed as an amendment to an army appropriation bill following the end of Reconstruction and was subsequently amended in 1956 and 1981.

As originally enacted in 1878, the Posse Comitatus Act applied only to the U.S. Army but was amended in 1956 to include the Air Force. In addition, the Department of the Navy has enacted regulations intended to apply the Posse Comitatus Act restrictions to the U.S. Navy and Marine Corps.

The Posse Comitatus Act does not apply to the Army National Guard and the Air National Guard when acting in a law enforcement capacity within its own state when ordered by the governor of that state or in an adjacent state if invited by that state’s governor.

Operating under the Department of Homeland Security, the U.S. Coast Guard is not covered by the Posse Comitatus Act. While the Coast Guard is an “armed service,” it also has both a maritime law enforcement mission and a federal regulatory agency mission.

The Posse Comitatus Act was originally enacted due to the feeling of many members of Congress at the time that President Abraham Lincoln had exceeded his authority during the Civil War by suspending habeas corpus and creating military courts with jurisdiction over civilians.

It should be noted that the Posse Comitatus Act greatly limits, but does not eliminate the power of the President of the United States to declare "martial law," the assumption of all civilian police powers by the military.

The president, under his or her constitutional powers to put down insurrection, rebellion, or invasion, may declare martial law when local law enforcement and court systems have ceased to function. For example, after the bombing of Pearl Harbor on December 7, 1941, President Roosevelt declared martial law in Hawaii at the request of the territorial governor.

What the National Guard Can Do on the Border

The Posse Comitatus Act and subsequent legislation specifically prohibit the use of the Army, Air Force, Navy and Marines to enforce the domestic laws of the United States except when expressly authorized by the Constitution or Congress. Since it enforces maritime safety, environmental and trade laws, the Coast Guard is exempt from the Posse Comitatus Act.

While Posse Comitatus does not specifically apply to the actions of the National Guard, National Guard regulations stipulate that its troops, unless authorized by Congress, are not to take part in typical law enforcement actions including arrests, searches of suspects or the public, or evidence handling.

What the National Guard Cannot Do on the Border

Operating within the limitations of the Posse Comitatus Act, and as acknowledged by the Obama administration, National Guard troops deployed to the Mexican Border States should, as directed by the states' governors, support the Border Patrol and state and local law enforcement agencies by providing surveillance, intelligence gathering, and reconnaissance support. In addition, the troops will assist with "counternarcotics enforcement" duties until additional Border Patrol agents are trained and in place. The Guard troops may also assist in the construction of roads, fences, surveillance towers and vehicle barriers necessary to prevent illegal border crossings.

Under the Defense Authorization Act for FY2007 (H.R. 5122), the Secretary of Defense, upon a request from the Secretary of Homeland Security, can also assist in preventing terrorists, drug traffickers, and illegal aliens from entering the United States.

Where Congress Stands On the Posse Comitatus Act

On Oct. 25, 2005, the House of Representatives and Senate enacted a joint resolution (H. CON. RES. 274) clarifying Congress' stance on the effect of the Posse Comitatus Act on the use of the military on U.S. soil. In part, the resolution states "by its express terms, the Posse Comitatus Act is not a complete barrier to the use of the Armed Forces for a range of domestic purposes, including law enforcement functions, when the use of the Armed Forces is authorized by Act of Congress or the President determines that the use of the Armed Forces is required to fulfill the President's obligations under the Constitution to respond promptly in time of war, insurrection, or other serious emergency."

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Longley, Robert. "Posse Comitatus Act and the US Military on the Border." ThoughtCo, Aug. 26, 2020, thoughtco.com/posse-comitatus-act-military-on-border-3321286. Longley, Robert. (2020, August 26). Posse Comitatus Act and the US Military on the Border. Retrieved from https://www.thoughtco.com/posse-comitatus-act-military-on-border-3321286 Longley, Robert. "Posse Comitatus Act and the US Military on the Border." ThoughtCo. https://www.thoughtco.com/posse-comitatus-act-military-on-border-3321286 (accessed June 4, 2023).