Posse Comitatus Act, 1878 Whoever, except in cases and under circumstances expressly authorized by the Constitution or Act of Congress, willfully uses any part of the Army or the Air Force as a posse comitatus or otherwise to execute the laws shall be fined under this title or imprisoned not more than two years, or both. -Title 18, U.S. Code,Section 1385
Modern Exceptions: Title 18 U.S. Code, Section 831, provides that if nuclear material is involved in an emergency, the Secretary of Defense may provide assistance to the Department of Justice, notwithstanding the Posse Comitatus Act.
Title 10 U.S. Code, Chapter 18, authorizes military support for civilian law enforcement agencies for counterdrug operations and in emergencies involving chemical or biological weapons of mass destruction. The Secretary of Defense may provide information, allow the use of military equipment and facilities, train law enforcement officials in the operation and maintenance of military equipment, and maintain such equipment. Support for law enforcement agencies may not impair military readiness, and military personnel shall not participate in searches, seizures, arrests, or similar activities unless such participation is otherwise authorized by law. (Military police personnel, for example, may enforce the law within their jurisdictions.)