Comment by dragonwriter

The Department of Defense was named in 1949, not 1947, and the thing that it was renamed from was the National Military Establishment, which was newly created in 1947 to be put over the two old military departments (War, which was over the Army only, and Navy, which was over the Navy including the Marine Corps)

At the same time as the NME was created, the Army was split into the Army and Air Force and the Department of War was also split in two, becoming the Department of the Army and the Department of the Air Force.