September 18, 1947 – After years as a branch of the Army, known as the U.S. Army Air Corps and later the U.S. Army Air Forces, the United States Air Force is formally founded as a seperate and autonomous Military Service.  The USAF falls under the Department of Defense and is the most recent branch of the United States Military to be formed. 

The National Security Act of 1947 states that the purpose of the Air Force is:

In general the United States Air Force shall include aviation forces both combat and service not otherwise assigned. It shall be organized, trained, and equipped primarily for prompt and sustained offensive and defensive air operations. The Air Force shall be responsible for the preparation of the air forces necessary for the effective prosecution of war except as otherwise assigned and, in accordance with integrated joint mobilization plans, for the expansion of the peacetime components of the Air Force to meet the needs of war.

This purpose is further defined in §8062 of Title 10 US Code (10 USC 8062) as:

• To preserve the peace and security, and provide for the defense, of the United States, the Territories, Commonwealths, and possessions, and any areas occupied by the United States;
• To support national policy;
• To implement national objectives;
• To overcome any nations responsible for aggressive acts that imperil the peace and security of the United States.

The stated mission of the USAF today is to “deliver sovereign options for the defense of the United States of America and its global interests — to fly and fight in Air, Space, and Cyberspace“.