Zhao swapped its defensive general for a bookman — and lost 400,000 soldiers at Changping

By 260 BCE the states of Qin and Zhao had been camped against each other at Changping for over two years. Zhao's veteran commander Lian Po had built fortified lines behind the Dan River and refused to give battle. The stalemate cost both sides heavily in food and men, but it was costing Qin — the attacker, far from home — more. Lian Po's logic was simple: Zhao could not win a pitched battle against Qin's army, but it could outlast it.

Qin sent spies into Zhao to spread the rumour that Lian Po was old and afraid, and that Qin feared only the young scholar-general Zhao Kuo. King Xiaocheng of Zhao, impatient with a war that would not end and pressed by the cost of feeding a standing army, believed the rumour. He dismissed Lian Po and gave command to Zhao Kuo — a man who had studied the military classics exhaustively but had never led troops in the field.

Zhao Kuo took command, tore up Lian Po's defensive plan, and marched the whole army across the Dan River to force a decisive battle. Qin had secretly promoted Bai Qi to command. Bai Qi feigned retreat, drew Zhao's army into a river valley, then sent 25,000 men around behind the Zhao lines to cut the supply route and 5,000 cavalry to sever the rear camp. The Zhao army, split and surrounded, held out for forty-six days until its men were starving. Zhao Kuo led a breakout and was killed by arrows.

The surviving Zhao soldiers, perhaps 400,000 of them, surrendered. Bai Qi ordered them buried alive; only 240 of the youngest were sent home to spread the terror. Zhao never recovered the manpower. A generation of fighting-age men was gone, and the state that had been Qin's only equal on the battlefield was broken. Qin unified China within forty years.