The Department of War was not renamed the “Department of Defense”. Up to and during the Second World War, the Department of War and Department of the Navy were

The Department of War was not renamed the “Department of Defense”. Up to and during the Second World War, the Department of War and Department of the Navy were both cabinet level departments, with the DoW having control of the Army and the DoN having control of the Navy. The National Security Act of 1947 reorganized the US military by splitting the Department of War into the Department of the Army and the Department of the Air Force and making those two departments, as well as the Department of the Navy, subordinate to an overall Department of Defense. (This was also when the Air Force became a separate branch from the Army.)