The United States Air Force (USAF) is the aerial warfare branch of the United States armed forces and one of the seven uniformed services. It was a part of the United States Army until September 18, 1947. The USAF is the largest and most of technologically advanced air force in the world. It has thousands of aircraft in service including bombers, fighter aircraft, transports and other kinds, and hundreds of Intercontinental ballistic missiles. Hundreds of thousands of men and women are on active duty in the Air Force and others are in the Reserves and the United States National Guard.