Posse Comitatus Act

The Posse Comitatus Act, enacted in 1878 and codified at 18 U.S.C. section 1385, generally prohibits the use of federal military forces to carry out civilian law enforcement within the United States unless expressly authorized by the Constitution or an act of Congress. The law reflects a longstanding principle that domestic policing is a civil rather than a military function. It does not by its terms restrict the National Guard when serving under state control, and it can be overridden by statutes such as the Insurrection Act. In 2025, a federal court ruled that the Trump administration’s use of federalized National Guard troops for immigration enforcement in Los Angeles violated the Act.