I wondered about that part too, and thought of: 1) America is neutralized by a massive nuclear strike; or: 2) Yellowstone blows apart, covering a large portion of the Lower 48 with volcanic ash.
I don’t think there is anyone on the planet that can take out america except america...at least not without some form of treachery, an ally stabs us in the back for instance, or a horrible president invites the UN to run our government. I think we will somehow do ourselve in. Probably a combination of economic disasters, natural disasters, political and corporate corruption, and liberalism.