Ah wheel chair generals, you are everywhere aren't you.
My take: Absolutely we could be invaded. But I doubt they could stay too long. While Americans aren't as patriotic as you foreign guys take us (remember, our major tv news is mostly watched by older folk) I'm sure a lot of gorilla groups would appear very quickly.
Secondly, americans hate change. We ***** when the gas rises. We ***** when a store doesn't sell what we want. We ***** when a show changes. We ***** when something is too hard. If we were invaded than transportation would be severely cut down, inflation would be massive for a short time, ect. People would get very pissed much faster than people in iraq did (excluding cultural differences at the time)
Thirdly, say what you want but Americans can be pretty damn crafty. Just look at some of our lawsuits - while we aren't the biggest user, the vast majority of people can connect to the internet which would supply enough information to get a local insurgency going.
Fourthly, americans are pretty damn violent at times. We aren't blood thirsty killers, but most men here can get defensive over anything. Matched up with our growing aggression towards outsiders, that wouldn't make a good combo for a large invading force.
Fifthly, while we do have a very large gun supply any invading force would be quick to destroy gun ships/ammo plants. But, we have a country right below us who are not only quite good at getting illegal stuff over the border, but the know-how to do/sell it quickly.
If any invading country..invaded, they could probably push themselves in before we got our shit together. The american military is pretty good, but it has it's limits.
EDIT: Oh yes, the nuclear weapons. I doubt any country would use nuclear weapons against us - simply too many targets to hit at the same time (including subs, allies should we have them, ect) to guarantee none are shot back over.