Posse Comitatus

The Posse Comitatus Act is a United States federal statute, originally enacted on June 18, 1878, that restricts the use of the U.S. Army—and, by later amendment and Department of Defense policy, the Air Force, Navy, Marine Corps, and Space Force—from executing domestic civilian laws unless expressly authorized by the Constitution or an Act of Congress. It is currently codified at 18 U.S.C. § 1385. The term itself comes from the Latin posse comitatus, meaning "force of the county," which historically referred to a sheriff's power to summon civilians to assist in keeping the peace.

The Act was passed at the end of Reconstruction, in part as a response to Southern Democratic objections to the use of federal troops to supervise elections and enforce civil rights in the former Confederate states. Its practical effect is to create a strong default presumption against military involvement in civilian law enforcement, while leaving room for statutory exceptions.

Key exceptions include:

• The Insurrection Act (10 U.S.C. §§ 251–255), which allows the President to deploy federal forces to suppress insurrection, enforce federal law, or protect civil rights when state authorities cannot or will not.
• National Guard forces operating under state command (Title 32 status), which are not bound by the Act because they are not in federal service.
• Specific statutory authorities permitting military support for counter-drug operations, border surveillance, and disaster response, generally limited to logistical or advisory roles.
• The Coast Guard, which has independent law-enforcement authority.

The doctrine is frequently invoked in debates over domestic deployment of troops, including during the 1992 Los Angeles riots, the 2020 protests following the death of George Floyd, and recurring discussions about militarization of the U.S.–Mexico border. Although the Act is U.S.-specific, the underlying civil–military principle—that armies should not police citizens—is a common feature of democratic governance and appears, in different forms, in many constitutional systems.