High Quality Content by WIKIPEDIA articles! The Muslim conquest of Persia (Persian: ?????? ????? hamle-ye a'rab "the attack of the Arabs" or ???? ????? zohur-e eslam "the dawn of Islam". The latter being officially preferred since the Iranian revolution) led to the end of the Sassanid Empire in 644, the fall of Sassanid dynasty in 651 and the eventual decline of the Zoroastrian religion in Persia. Arabs first entered Sassanid territory in 633, when general Khalid ibn Walid invaded what is now Iraq. Following the transfer of Khalid to the Roman front in the Levant, the Muslims eventually lost their holdings to Persian counterattacks. The second invasion began in 636 under Saad ibn Abi Waqqas, when a key victory at the Battle of Qadisiyyah led to the permanent end of Sassanid control west of Persia. The Zagros mountains then became a natural barrier and border between the Rashidun Caliphate and the Sassanid Empire. Owing to continuous raids by Persians into the area, Caliph Umar ordered...