What is currently considered the DoD was built after WW2 as the "National Military Establishment" by the "National Security Act of 1947" which restructured and reformed significant war and military assets under the "Secretary of Defense" and the NME was very quickly renamed the "Department of Defense".
The "Department of War" during WW2 was in control of the Army, and was separate from the Department of the Navy and eventual Department of the Air Force (spun off from the Army) and was headed by the "Secretary of War".
Changing the name to "Defense" was an intentional act by a President and government who wanted to reduce the power of the Military Industrial Complex and reduce the "War" focus of a subset of the government, and force the different departments to work together and share toys.
The reorganization was desired for many reasons but Truman made lots of talk about how this was about the national defense and made gestures to the Pearl Harbor attack as something relevant. Different departments failing to work together was a huge problem during WW2, and other wars. Putting them all under one single cabinet position, the Secretary of Defense, was a significant point.
This vocab was used during the war, about the reorganization being about the defense of the nation.
Similarly, NATO is a defense only pact, in very clear terms.
There was tons of debate in the US government at the time as to whether we had viable intelligence of the Japanese attack at Pearl Harbor ahead of time and it wasn't properly utilized or disseminated. In fact, there were many such instances in the Pacific Theater early on, where poor intelligence handling resulted in worse battle outcomes.
The point of the Department of Defense is to Defend America, and they do that by being in control of our Military. Letting our defense assets bully the world is the Utter Failure of the American voting public over the past 100 years.
