Department of Defense has historically been a prime example of newspeak.

I think Department of War is also newspeak. Or at least, they didn't change the name just to get the name in line with the amount of war the department does.

They changed it because they wanted to do more even more war. The amount of war the department does under the name "Defense" has been status quo for a long time, and my take is they wanted us to think of them differently so they could do even more war, which they have since been doing.