When was America last invaded by a foreign adversary?
Yes, invading Hawaii was part of imperial Japanese planning. If you don’t understand that defense spending is still worthwhile even if you don’t blow anything up with it, I’m not sure how we connect.
You're basically saying the US don't need a Department of Defense because the Department of War is doing such a good job.
How do you know that's the reason? There are many countries with very low defense spending that haven't been invaded since WW2.