The United States Air Force (USAF) is the aerial warfare service branch of the United States Armed Forces and one of the eight American uniformed services. Initially part of the United States Army, the USAF was formed as a separate branch of the military on 18 September 1947 under the National Security Act of 1947. It is the second most recent branch of the U.S. military to be formed, and is the largest and one of the world's most technologically advanced air forces.