The American Revolution
Thirteen British colonies broke away to found a republic on a radical claim — that legitimate power flows from the consent of the governed, not from a king or God. Their success turned an Enlightenment idea into a working government and lit a fuse under every monarchy watching. It is a founding case of a legitimating narrative collapsing: the divine and imperial right to rule giving way to popular consent, an idea Europe was about to test far more violently.
The story that justifies an order (divine right, mandate, ideology) loses credibility; the order it propped up follows.