Prithviraj won at Tarain and let the sultan go — the next year he lost his kingdom

In 1191 Sultan Muhammad of Ghor — Mu'izz al-Din, the Ghurid ruler pushing east from Afghanistan — met the Chauhan king Prithviraj at Tarain, near Delhi. The Rajput army won the field; the sultan was wounded and retreated. Prithviraj did not pursue. The chronicles argue over why — chivalry, exhaustion or confidence — but the result was that a beaten enemy rode away intact.

Muhammad spent the year rebuilding around the lesson. In 1192 he returned to Tarain with a larger force and different tactics: mounted archers who feigned retreat, broke the Rajput charges, then turned on them. Prithviraj's army was routed, the king was captured and killed, and Ajmer and Delhi fell. Ghurid rule was planted in northern India.

The first Tarain was won by preparing for the war that was coming; the second was lost by preparing for the war that had already happened. Prithviraj's defence was built for the 1191 battle — massed charges against a weakened invader — while his enemy spent the intervening year designing the answer to exactly that.