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The Posse Comitatus Act is a United States federal law (18 U.S.C.) passed in 1878 after the end of Reconstruction.
Posse Comitatus clarifications emphasize supportive and technical assistance (e.g., use of facilities, vessels, aircraft, intelligence, tech aid, surveillance) while generally prohibiting direct participation of Department of Defense personnel in law enforcement (e.g., search, seizure, and arrests).
Section 1385 of title 18 (commonly known as the Posse Comitatus Act) prohibits the use of the Armed Forces as a Posse comitatus to execute the laws except in cases and under circumstances expressly authorized by the Constitution or Act of Congress.