The Posse Comitatus Act of 1878
The Posse Comitatus Act of 1878 (PCA) prohibits the use of the Army or the Air Force as a posse comitatus to execute the laws of the U.S.
"POSSE COMITATUS ACT" (18 USC 1385): A Reconstruction Era criminal law proscribing use of Army (later, Air Force) to "execute the laws" except where expressly authorized by Constitution or Congress. Limit on use of military for civilian law enforcement also applies to Navy by regulation. Dec '81 additional laws were enacted (codified 10 USC 371-78) clarifying permissible military assistance to civilian law enforcement agencies--including the Coast Guard—especially in combating drug smuggling into the United States. Posse Comitatus clarifications emphasize supportive and technical assistance (e.g., use of facilities, vessels, aircraft, intelligence, tech aid, surveillance, etc.) while generally prohibiting direct participation of DoD personnel in law enforcement (e.g., search, seizure, and arrests). For example, Coast Guard Law Enforcement Detachments (LEDETS) serve aboard Navy vessels and perform the actual boardings of interdicted suspect drug smuggling vessels and, if needed, arrest their crews). Positive results have been realized especially from Navy ship/aircraft involvement. While debaters debate, JHOC employs the strengths of USCG and USN to provide an effective harbor security solution. For example the Navy is not in the business of commercial port operation, and left to its own devices may impose a more stringent 'stop and check' of everything everything moving in the harbor, effectively stymieing commerce in the region.
However, the Navy does have much "deeper pockets" than the Coast Guard, and of course a superior number of advanced boats and firepower. For its part, the Coast Guard delivers its port operations experience.
In the event of a valid threat, the USCG is able to physically commandeer naval resources to adequately meet and address it. The result is maximum protection of high value naval and commercial targets, with the continuation of commerce in "business as usual" mode. "While we are responsible for our ships and our Navy ports, the Coast Guard is in charge of the entire port of Hampton Roads," said Capt. Joseph F. Bouchard, the commanding office or Naval Station Norfolk. "By working together we can accomplish more with the resources available to each agency. We can track ships throughout the harbor. It greatly extends our defensive perimeter. Now we can detect potential threats before they can become a problem off the naval station piers," he said.