From the Coast Guard Website
The United States Coast Guard, one of the country's five armed services, is also one of the most unique agencies of the federal government. We trace our history back to 4 August 1790, when the first Congress authorized the construction of ten vessels to enforce tariff and trade laws, prevent smuggling, and protect the collection of the federal revenue. Known variously as the Revenue Marine and the Revenue Cutter Service, we expanded in size and responsibilities as the nation grew.
These added responsibilities included humanitarian duties such as aiding mariners in distress. Our law enforcement functions also continued to expand. Congress tasked us with enforcing laws against slavery, piracy, and enlarged our responsibilities to prevent smuggling. We were also given the responsibility to protect the marine environment, explore and police Alaska, and chart the growing nation's coastlines, all well before the turn of the twentieth century.
The service received its present name in 1915 under an act of Congress when the Revenue Cutter Service merged with the Life-Saving Service. The nation now had a single maritime service dedicated to saving life at sea and enforcing the nation's maritime laws. We began to maintain the country's aids to maritime navigation, including operating the nation's lighthouses, when the Lighthouse Service was transferred to the Coast Guard in 1939. Later, in 1946, Congress permanently transferred the Bureau of Marine Inspection and Navigation to the Coast Guard, thereby placing merchant marine licensing and merchant vessel safety under our purview.
The Coast Guard is one of the oldest organizations of the federal government and, until the Navy Department was established in 1798, we served as the nation's only armed force afloat. We have continued to protect the nation throughout our long history and have served proudly in every one of the nation's conflicts. Our national defense responsibilities remain one of our most important functions even today.
In times of peace we operate as part of the Department of Homeland Security, serving as the nation's front-line agency for enforcing our laws at sea, protecting our coastline and ports, and saving life. In times of war, or on direction of the President, we serve under the Navy Department.