<human>:  When did the US Air Force become a separate branch of the military?
<bot>: The US Air Force was established as a separate branch of the military in 1947. The Air Force is the second largest branch of the US military, behind the Army. The Air Force was formed as a result of World War II, and was originally part of the Army Air Corps. The Air Force was created to be the nation’s first line of defense against air attacks. The Air Force is responsible for airspace defense, strategic bombing, and providing aerial refueling and combat support to other branches of the military.