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.
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Economic collapse, maybe. People here have become weaker over the decades. Everybody want everybody else to look out for them. I love America, but the people are suit-happy, which is also a serious threat.