Perhaps we have different definitions of "civilization-ending". Even if what you describe here actually happened, I wouldn't characterize that as a civilization-ending event. it would just be the passing of another nation. Civilization would continue, just as it has when nations have fallen in history.

Although, I do need to add, while your scenario is technically possible, I do think it's vanishingly unlikely. The US has been down that road before and survived, as have many others.