Assuming it doesn't get assimilated into some sort of super-country in the future I think it'll end up tearing itself apart, or splitting some time in the future (assuming it splits in the [relatively] near future). Democracy doesn't hold such a large and diverse nation together very well. The invasion of a foreign country isn't going to take us down; not soon at least. It would only serve to strengthen nationalism and bring people together, and America has a lot of big explody thingies that work pretty effectively when people are rallied towards a single, indisputable cause.