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.

Often offensive and also often defensive of others.. so if renaming is on the table, it’s probably most apt to call it the Dept of Security since the vast majority of what it does is maintaining the security umbrella that has helped suppress world war since the last one. Of course, facts or opinions on whether it succeeds on the security front depend on which side of the umbrella you’re on.

And losing at that offense while at it.