The mission of the US Department of the Air Force is to fly, fight, and win -- in air, space, and cyberspace. Along with the Army, Navy, and Marine Corps, the US Air Force is a major military branch of the US Department of Defense responsible for defending the US and its interests through aerial, space, and cyber warfare. The agency includes more than a dozen major commands, including the Air Force Reserve and Air National Guard. It also consists of field operating agencies and direct reporting units, which are typically assigned to specialized missions. Originally part of the US Army, the US Air Force was formed as a separate branch when president Harry S. Truman signed the National Security Act of 1947.