The Battle of Nahavand ( , ), also spelled Nihavand or Nahawand, was fought in 642 between the Rashidun Muslim forces under caliph Umar and Sasanian Persian armies under King Yazdegerd III. Yazdegerd escaped to the Merv area, but was unable to raise another substantial army. It was a victory for the Rashidun Caliphate and the Persians consequently lost the surrounding cities including Spahan (Isfahan). The former Sassanid provinces, in alliance with Parthian and White Hun nobles, resisted for about a century in the region south of the Caspian Sea, even as the Rashidun Caliphate was replaced by the Umayyads, thus perpetuating the Sassanid court styles, Zoroastrian religion, and Persian language. Background At the time of the death of the Islamic Prophet Muhammad in 632, the religion that he led dominated the Hejaz (western Arabia). Under the first two caliphs Abu Bakr and Umar, Islam expanded into Palestine and Mesopotamia where it respectively confronted the East Roman and Persian (Sāsānian) empires. Both were exhausted by warfare and internal dissent. With the East Roman defeat at the Battle of Yarmouk (636), the Muslim Arabs were free to turn east to the Euphrates and the Persian heartland. In November 636 a Sāsānian army was defeated at the Battle of Qadisiya, resulting in the loss of Iraq to the Muslims. Number of Arabs and Sasanian forces Following defeat by the Arabs in 639, the "King of Kings" Yazdgerd III was forced to abandon his capital at Ctesiphon. From Mesopotamia, he withdrew into the Sāsānian homeland in what is now the southern plateau of Iran. There he was able to assemble an army to replace that lost at Qadisiya. At Nahāvand, an estimated 30,000 Arab warriors under the command of Nuʿmān attacked a Sāsānian army reportedly of ca. 100,000 men. According to Peter Crawford, the forces that gathered there is variously recorded as anything from 50,000 to 100,000. Mostly farmers and villagers, not veteran soldiers. The Sāsānian troops, commanded by Fīrūzan, were entrenched in a strong fortified position. After an indecisive skirmish, Nuʿmān pretended to be defeated and withdrew from the battlefield. Fīrūzan then abandoned his position and pursued his foe. The pursuit proved to be a major tactical error because the Sāsānians were forced to fight on unfavourable ground; the Sāsānian army, caught between two mountain defiles, bravely fought for two days and three nights, after heavy casualties by both sides, iranians were