One of the five armed services and the only one not in the Department of Defense. In peacetime, they are part of the Department of Homeland Security because of their missions which include water search and rescue, drug interdiction and waterway safety. (background) Prior to being transferred to Homeland Security, they were part of the Department of Transportation. They were originally part of the Treasury Department because one of the major components that became the Coast Guard, the Revenue Cutter Service, was in that Department. The other major component that became the Coast Guard in the early 20th Century was the coastal US Life Saving Service.