The Mongol Sack of Baghdad
The Mongols reached Baghdad, the jewel of the Islamic world, and destroyed it — killing hundreds of thousands, executing the caliph, and hurling the vast libraries of the House of Wisdom into the Tigris until, it was said, the river ran black with ink. Five centuries of accumulated learning and the political heart of the Islamic golden age were extinguished in days. It is one of history's sharpest reminders that the patient work of civilization can be undone far faster than it was built.
A sudden collapse in population (plague, famine, mass violence) creates labor scarcity that reorders economy and society.