Exceptions
There are a number of exceptions to the act. These include:
;National Guard units while under the authority of the governor of a state
Troops when used pursuant to the Federal authority to quell domestic violence as was the case during the 1992 Los Angeles riots;
The President of the United States can waive this law in an emergency;
In December 1981 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) while generally prohibiting direct participation of Department of Defense 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.
Under 18 USC 831, the Attorney General may request that the Secretary of Defense provide emergency assistance if civilian law enforcement is inadequate to address certain types of threat involving the release of nuclear materials, such as potential use of a Nuclear or Radiological weapon. Such assistance may be by any personnel under the authority of the Department of Defense, provided such assistance does not adversely affect US military preparedness.
