What is Necessary for Legitimate Self-Defense

I had a conversation earlier today about the military force of the United States of America, and I made the assertion that the USA could provide for its legitimate self-defense needs with about 10% of the current military spending.

What do you think about the legitimate self-defense needs of the USA? I am not talking about defending imperialism. Just regular old basic self-defense. What's reasonable?

My understanding is that military, and military related, spending by the federal government is now up over the $1T mark (that is one trillion dollars: $1,000,000,000,000 (yes, that is 12 zeros.))

I wonder if the USA could provide for its legitimate needs for self-defense on a far slimmer budget. Yes, I think it could. Sounds like a good idea to me. By spending less on military - 9/10 of which (I postulate) is unnecessary and unrelated to legitimate self-defense - more money would be available to promote lasting and sustainable economic endeavors, broadening opportunity, and prosperity for all.

As far as imperialism goes, imperialism is actually anathema to national security - it's a destabilizing economic influence. With imperialism, although some may (perhaps) benefit — a great many others suffer, their well-beings (collective and individual) jeopardized and harmed.