Air Defense GP is an acronym for Strategic Air Command (SAC). SAC was a Major Command of the United States Air Force, in existence from 1946 to 1992. It was responsible for the organization and training of US air forces, and for the defense of the United States against air attack. SAC also had responsibility for the nuclear deterrence mission of the United States. During its existence, SAC was the largest and most powerful air force in the world.