"The language of the [Posse Comitatus] Act mentions only the Army and the Air Force, but it is applicable to the Navy and Marines by virtue of administrative action and commands of other laws. The law enforcement functions of the Coast Guard have been expressly authorized by act of Congress and consequently cannot be said to be contrary to the Act. The Act has been applied to the National Guard when it is in federal service, to civilian employees of the armed forces, and to off-duty military personnel.
"The Act is probably only applicable within the geographical confines of the United States, but the supplemental provisions of 10 U.S.C. 371-381 appear to apply world-wide. Finally, the Act is a criminal statute under which there has never been a prosecution. Although violations will on rare occasions result in the exclusion of evidence, the dismissal of criminal charges, or a civil cause of action, as a practical matter compliance is ordinarily the result of military self-restraint."
Doyle, Charles, "The Posse Comitatus Act and Related Matters: The Use of the Military to Execute Civilian Law," Congressional Research Service (Washington, DC: Library of Congress, June 1, 2000), Summary.