So much historical ignorance. There was no single predecessor. We did not have a unified department prior to the Department of Defense; we had the Department of War (Army) and the Department of the Navy. At the end of World War II, it was decided to split off the Army Air Forces as a separate service. And, rather than have a third military Department of the Air Force, it was decided to put them all together in a unified Department of Defense.
So much historical ignorance. There was no single predecessor. We did not have a unified department prior to the Department of Defense; we had the Department of War (Army) and the Department of the Navy. At the end of World War II, it was decided to split off the Army Air Forces as a separate service. And, rather than have a third military Department of the Air Force, it was decided to put them all together in a unified Department of Defense.