There's gotta be a decent-sized amount of those Americans who secretly... Hell, maybe publicly... wish we were the ones being invaded, just so they could fulfill their dreams and have a sense of justification for why they're armed better than most third world countries.
I will say this is one of those rare scenarios where our country's citizens being more heavily armed than any others' would be a good thing. Of course, in the list of reasons that the U.S. is difficult to invade, there's quite a few you'd go through before you got to that fact.