Battle of Nahavand

The Battle of Nahāvand (also Nihāvand or Nahāwand) (Arabic:معركة نهاوند) Battle of Nahāwand was fought in 642 between Arab Muslims and Sassanid armies. The battle is known to Muslims, as the "Victory of Victories." William Durant in his book The Age of Faith reported that the Persian King Yazdgerd III had about 150,000 men, versus a Muslim army about one fifth that in number. The Persians were outmaneuvered, trapped in a narrow mountain valley, and lost approximately 100,000 men in the ensuing rout. Yezdigerd hurriedly fled to the Merv area, but was unable to raise another substantial army. It was a decisive victory for the Rashidun Caliphate and the Persians consequently lost the surrounding cities including Sephahan (renamed Isfahan). The Khan of the Turks later lent him some soldiers, but the soldiers mutinied in the year 652. Tabari reported that the Persian Sassanid army consisted mostly of raw recruits and men who had not fought in any previous battles.

http://en.wikipedia.org/wiki/Battle_of_Nihawand