Posse Comitatus and Domestic Deployment

After federal troops were used to supervise Southern elections and enforce laws during Reconstruction, Congress passed the Posse Comitatus Act, signed on June 18, 1878, which makes it a crime to use the Army to execute the laws except where expressly authorized by the Constitution or an act of Congress. The law now covers the Army, Air Force, and Space Force, and Defense Department policy applies the same limits to the Navy and Marine Corps. The Coast Guard, which has law enforcement duties, is not covered.

The National Guard occupies a special position. When under a governor's control, either on state active duty or in federally funded Title 32 status, Guard troops are not covered by the act and may perform law enforcement under state law. When called into federal service under Title 10, they are subject to it. Exceptions allow troops to provide support such as equipment, training, and surveillance to law enforcement, to respond to certain emergencies, and to suppress insurrections under the Insurrection Act. Federal law also lets the President federalize the Guard to repel invasion, suppress rebellion, or execute federal laws when regular forces are unable.

In June 2025 the President federalized California National Guard troops and sent Marines to Los Angeles over the governor's objection during protests against immigration enforcement, and a federal judge later ruled that their use violated the Posse Comitatus Act. Guard troops were also deployed in Washington, D.C., in August, and deployments to Portland, Chicago, and other cities that fall drew lawsuits by states and cities and conflicting court rulings. Supporters said the deployments protected federal personnel and property and reduced crime, while critics said they militarized domestic law enforcement and violated state authority and federal law.