Abu Bakr defeated the Byzantine army at Damascus in 635 and then began his conquest of Iran. In 637 the Arab forces occupied the Sassanid capital of Ctesiphon (which they renamed Madain), and in 641-42 they defeated the Sassanid army at Nahavand. After that, Iran lay open to the invaders. The Islamic conquest was aided by the material and social bankruptcy of the Sassanids; the native populations had little to lose by cooperating with the conquering power. Moreover, the Muslims offered relative religious tolerance and fair treatment to populations that accepted Islamic rule without resistance. It was not until around 650, however, that resistance in Iran was quelled. Conversion to Islam, which offered certain advantages, was fairly rapid among the urban population but slower among the peasantry and the dihqans [farmers]. The majority of Iranians did not become Muslim until the ninth century. Although the conquerors, especially the Umayyads (the Muslim rulers who succeeded Mohammad from 661-750), tended to stress the primacy of Arabs among Muslims, the Iranians were gradually integrated into the new community. The Muslim conquerors adopted the Sassanid coinage system and many Sassanid administrative practices, including the office of vizier, or minister, and the divan, a bureau or register for controlling state revenue and expenditure that became a characteristic of administration throughout Muslim lands. Later caliphs adopted Iranian court ceremonial practices and the trappings of Sassanid monarchy. Men of Iranian origin served as administrators after the conquest, and Iranians contributed significantly to all branches of Islamic learning, including philology, literature, history, geography, jurisprudence, philosophy, medicine, and the sciences. The Arabs were in control, however. The new state religion, Islam, imposed its own system of beliefs, laws, and social mores. In regions that submitted peacefully to Muslim rule, landowners kept their land. But crown land, land abandoned by fleeing owners, and land taken by conquest passed into the hands of the new state. This included the rich lands of the Sawad, a rich, alluvial plain in central and southern Iraq. Arabic became the official language of the court in 696, although Persian continued to be widely used as the spoken language. The shuubiyya literary controversy of the ninth through the eleventh centuries, in which Arabs and Iranians each lauded their own and denigrated the other's cultural traits, suggests the survival of a certain sense of distinct Iranian identity. In the ninth century, the emergence of more purely Iranian ruling dynasties witnessed the revival of the Persian language, enriched by Arabic loanwords and using the Arabic script, and of Persian literature. Another legacy of the Arab conquest was Shia Islam, which, although it has come to be identified closely with Iran, was not initially an Iranian religious movement. It originated with the Arab Muslims. In the great schism of Islam, one group among the community of believers maintained that leadership of the community following the death of prophet Mohammad rightfully belonged to Mohammad's son-in-law, Ali, and to his descendants. This group came to be known as the Shiat Ali, the partisans of Ali, or the Shias. Another group, supporters of Muawiya (a rival contender for the caliphate following the murder of Uthman), challenged Ali's election to the caliphate in 656. After Ali was assassinated while praying in a mosque at Kufa in 661, Muawiya was declared caliph by the majority of the Islamic community. He became the first caliph of the Umayyad dynasty, which had its capital at Damascus. Ali's youngest son, Hossain, refused to pay the homage commanded by Muawiya's son and successor Yazid I and fled to Mecca, where he was asked to lead the Shias--mostly those living in present-day Iraq--in a revolt. At Karbala, in Iraq, Hossain's band of 200 men and women followers, unwilling to surrender, were finally cut down by about 4,000 Umayyad troops. The Umayyad leader received Hossain's head, and Hossain's death in 680 on the tenth of Moharram continues to be observed as a day of mourning for all Shias.