A Reconstruction-era American law that bars the armed forces from taking part in domestic law enforcement. The act can be overridden by the Insurrection Act, enacted in 1807, which allows a president to deploy the military domestically when "unlawful obstructions, combinations or assemblages, or rebellion against the authority of the United States, make it impractical to enforce the laws". It does not require the consent of a state's governor. In 2025 the Trump administration directed the defence department to take control of a strip of public land north of the border wall, a move some legal scholars see as an attempt to evade Posse Comitatus: if a migrant jumps the wall they would be trespassing on a military facility and could be apprehended without violating the act. Using the Insurrection Act for immigration enforcement would be unprecedented.
In August 2025 Trump deployed National Guard troops to Washington, DC, ostensibly to fight crime, alongside 500 agents from the FBI and other federal law-enforcement agencies. The troops were not making arrests; they collected rubbish and spread mulch in city parks. Troops were also sent to Los Angeles in June 2025 to quell immigration-related protests, accompanied by Immigration and Customs Enforcement (ICE) agents on raids. The Pentagon allowed ICE to use a navy base outside Chicago as a staging ground, and army lawyers were seconded to work as immigration judges. Trump diverted 28,000 federal law-enforcement personnel—including one in five FBI agents—to do ICE's work.
A district-court judge ruled the California deployment illegal; the ruling is on hold while the administration appeals. The Trump administration argues that the Posse Comitatus Act does not apply and that courts cannot second-guess the president's assessment of when a security threat justifies troops. It cites Martin v Mott, a Supreme Court ruling from the War of 1812, which gave the president absolute discretion to call up a militia: "The authority to decide whether the exigency has arisen belongs exclusively to the President, and that his decision is conclusive upon all other persons."
Charles Dunlap, then an air-force lawyer, warned 30 years ago of the increasing encroachment of the armed forces into civilian life, which he called a "subtle drift towards an uncertain destination".
On December 23rd 2025 the Supreme Court ruled 6-3 in Trump v Illinois that the president could not deploy National Guard troops in Chicago to manage protests against his immigration policy. The government had cited Section 12406 of Title 10, which allows the president to federalise Guard units when he is "unable with the regular forces to execute the laws". The court held, following an argument by Marty Lederman of Georgetown Law School, that "the regular forces" means active-duty military, not civilian law-enforcement agencies such as ICE. Amy Coney Barrett and Brett Kavanaugh joined the three Democratic appointees and Chief Justice John Roberts in the majority.
The ruling sent about 300 troops home and appeared to undermine deployments in other Democrat-led cities, including Los Angeles, New Orleans and Portland, Oregon.
The Insurrection Act has been invoked about 30 times in American history. Presidents have almost always used it with a governor's consent. The rare unilateral deployments involved state officials defying federal law, such as when Eisenhower sent troops to Arkansas to enforce school desegregation. Its sweeping language about rebellion makes invocation politically perilous. As Nicholas Katzenbach, a deputy attorney-general, wrote in 1964, the deployment of federal troops may "aggravate the emotions of the populace or alienate local law enforcement officials".
"They told me I was gullible ... and I believed them!"