Strife, assuming for a minute we DID get rid of everything but the navy+missile(nuke) power, how often would we NEED to have boots on the ground taking and holding ground? Our Navy alone seems to be more than enough to keep us completely secure from the threat of invasion AND act as a huge deterrent to those other conflicts, and I think its arguable that not having the option to do a full scale land invasion might have been better for us, economically, at this point.
Naval forces are great for power projection, but one can't win wars without boots. I'll bring up Desert Storm/Shield again, because, as far as I'm concerned, it's what we'd want in our Armed Forces, one that can field an Armored Corp strong enough to crush aggression without trying to occupy the country.
The United States, to some extent, supported both sides in the Iran-Iraq war. Moreover, I'd certainly claim that Iraq was the lesser of the two evils there, and our support was never all that much. The failure of Desert Storm was not one of the military, it was a political decision to let Iraq withdraw all their material. Even then, the actual invasion was a massacre. In an open battle, the US Armed forces *isn't* beatable. That's why you have Iraqi divisions surrendering to reporters. It's trying to hold ground against internal insurgency that we were bad at (note a very big difference from holding ground against other armies), and we're getting much, much, much better.
We didn't finish Desert Storm right, but we're finishing this one the right way, leaving a semi-stable Iraqi government that can take care of itself. I don't forsee us having to land an Army again here, excepting some other external action.
The U.S. can't be invaded. We have two huge oceans acting as barriers. Our land neighbors Canada and Mexico will never invade us. In order for China or Russia (which many people seem to view as a threat) to invade, they will need a large navy to get troops across the oceans. Maintaining a better Navy than the countries we're scared of might be worth it. As well, as a few nukes to provide the threat of mutual destruction. But at the moment we're really over doing it . . .
Both Canada and Mexico have invaded the US in the past. And we have invaded them. In Mexico there are still serious political movements that want to see their territories in Texas and California taken back. So I wouldn't exactly say "never".
Never say never, I guess. But in the medium-term future, say at least 10 years, I am confident that the US will not face invasion from within North America.