U.S. Navy Birthday, October 13, 1775. The United States Navy (USN) is the naval warfare service branch of the United States Armed Forces. The U.S. Navy also has the world's largest aircraft carrier fleet, with 317,054 personnel on active duty and 109,671 in the Navy Reserve. U.S. Navy traces its origins to the Continental Navy, which was established during the American Revolutionary War and was essentially disbanded as a separate entity shortly thereafter. It played a major role in the American Civil War by blockading the Confederacy and seizing control of its rivers. It played the central role in the World War II defeat of Japan.