I really don't know because it would depend on so many things. I think the two main options would be hide or fight. This is a big country and there are a lot of areas that are "the middle of now where" that might be good places to be because it wouldn't be worth going after you. The other option would be to reenact "Red Dawn," I guess and start our own insurgency. I'm trying to imagine how we get into this scenario. An invasion of a country this size is not going to be a quick thing; lots of people will die on both sides before we lose the war. Also, what political or other entity is the de facto government (holds the monopoly on the "legitimate" use of violence) and is thus the target of our insurgent attacks? Or is it basically a free-for-all of some sort with a lot of groups participating in the invasion, and maybe dividing up the country into various areas controlled by different armies, so there are a lot of little "governments"? I do think it's a good idea to consider various possibilities and what some of the responses could be.