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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Politicians taking the current political entrenchment to the extreme. Special interests bleeding the nation dry.
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It will never fall. It may eat itself and implode by taking away more and more freedoms creating a police state. America will always have a important and valuable role in the world. How they deal with that responsibility is what worries me.
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Heavily televised and with plenty of advertising.
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Socialism. Europe style.
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Cave in?
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Economically... wait...
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Even if something horrible happened, we are such a big part of the world economy that we would get bailed out by countries like China that need someone to sell their everything to.
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Liberals will ruin it. Actually, they already started.
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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.
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By becoming Europe.
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Down some stairs.
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Fallout Universe.
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Aliens