The United States Navy is the world's second-largest navy by vessel count (290 combat ships) but the largest by displacement, and it operates the largest fleet of aircraft carriers (11 in service). Originating from the Continental Navy in the American Revolutionary War, it became the world's dominant naval force in World War II, defeating the Imperial Japanese Navy, and later served as a nuclear deterrent during the Cold War. As part of the Department of the Navy alongside the Marine Corps, it is headed by the Secretary of the Navy, with the Chief of Naval Operations as its senior officer. The service fields over 336,000 active-duty personnel, 4,000+ aircraft, and 70 nuclear-powered submarines, making it the third-largest US military branch. Its Ohio-class submarines carry 1,895 nuclear warheads, representing 51% of the US nuclear stockpile and 19% of the global total. The Navy's core mission revolves around sea control, power projection, deterrence, maritime security, and sealift to ensure rapid response to regional crises worldwide.