Originally Posted by
KenV
The US Coast Guard is a military service, but not part of the Department of Defense. Coast Guard was originally part of the Department of Commerce, but recently was moved to Department of Homeland Security, which is a law enforcement agency. This is an important distinction under our Constitution. DoD assets may not be used for law enforcement duties. However, in times of war, the US Coast Guard can become part of the US Navy.
USCG does maritime law enforcement (fisheries laws, smuggling laws, maritime environmental protection laws, etc etc), operates and maintains waterways navaids (such as light houses, buoys, etc), and is responsible for maritime search and rescue. Apparently all those maritime law enforcement duties in the UK fall to the Royal Navy?
i don’t believe USCG has ever been part of the Department of Commerce. It was under the Department of Transportation for many years before being moved to Homeland Security after 9/11. As you say, it can be placed under the Navy with a Congressional decree.