The United States Air Force (USAF) is the air service branch of the United States Armed Forces and part of the civilian-led Department of the Air Force, which is led by the Secretary of the Air Force. The military head of the U.S. Air Force is the chief of staff of the Air Force, who is assisted by the vice chief of staff of the United States Air Force and chief master sergeant of the Air Force. It achieved independence on 18 September 1947 from the U.S. Army, but directly traces its history through the United States Army Air Forces, United States Army Air Corps, United States Army Air Service, the Division of Military Aeronautics, Aviation Section, U.S. Signal Corps, to the birth of Aeronautical Division, U.S. Signal Corps on 1 August 1907.