The beduin Arabs who toppled the Sassanid Empire were propelled not only by a desire for conquest but also by a new religion, Islam. The Prophet Muhammad, a member of the Hashimite clan of the powerful tribe of Quraysh, proclaimed his prophetic mission in Arabia in 612 and eventually won over the city of his birth, Mecca, to the new faith. Within one year of Muhammad's death in 632, Arabia itself was secure enough to allow his secular successor, Abu Bakr, the first caliph, to begin the campaign against the Byzantine and Sassanid empires.

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. The majority of Iranians did not become Muslim until the ninth century.

Although the conquerors, especially the Umayyads (the Muslim rulers who succeeded Muhammad 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 Muhammad rightfully belonged to Muhammad'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, Husayn, 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, Husayn'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 Husayn's head, and Husayn's death in 680 on the tenth of Moharram continues to be observed as a day of mourning for all Shias.

The largest concentration of Shias in the first century of Islam was in southern Iraq. It was not until the sixteenth century, under the Safavids, that a majority of Iranians became Shias. Shia Islam became then, as it is now, the state religion.

The Abbasids, who overthrew the Umayyads in 750, while sympathetic to the Iranian Shias, were clearly an Arab dynasty. They revolted in the name of descendants of Muhammad's uncle, Abbas, and the House of Hashim. Hashim was an ancestor of both the Shia and the Abbas, or Sunni, line, and the Abbasid movement enjoyed the support of both Sunni and Shia Muslims. The Abbasid army consisted primarily of Khorasanians and was led by an Iranian general, Abu Muslim. It contained both Iranian and Arab elements, and the Abbasids enjoyed both Iranian and Arab support.

Nevertheless, the Abbasids, although sympathetic to the Shias, whose support they wished to retain, did not encourage the more extremist Shia aspirations. The Abbasids established their capital at Baghdad. Al Mamun, who seized power from his brother, Amin, and proclaimed himself caliph in 811, had an Iranian mother and thus had a base of support in Khorasan. The Abbasids continued the centralizing policies of their predecessors. Under their rule, the Islamic world experienced a cultural efflorescence and the expansion of trade and economic prosperity. These were developments in which Iran shared.

Iran's next ruling dynasties descended from nomadic, Turkic-speaking warriors who had been moving out of Central Asia into Transoxiana for more than a millennium. The Abbasid caliphs began enlisting these people as slave warriors as early as the ninth century. Shortly thereafter the real power of the Abbasid caliphs began to wane; eventually they became religious figureheads while the warrior slaves ruled. As the power of the Abbasid caliphs diminished, a series of independent and indigenous dynasties rose in various parts of Iran, some with considerable influence and power. Among the most important of these overlapping dynasties were the Tahirids in Khorasan (820-72); the Saffarids in Sistan (867-903); and the Samanids (875-1005), originally at Bukhara (also cited as Bokhara). The Samanids eventually ruled an area from central Iran to India. In 962 a Turkish slave governor of the Samanids, Alptigin, conquered Ghazna (in present-day Afghanistan) and established a dynasty, the Ghaznavids, that lasted to 1186.

Several Samanid cities had been lost to another Turkish group, the Seljuks, a clan of the Oghuz (or Ghuzz) Turks, who lived north of the Oxus River (present-day Amu Darya). Their leader, Tughril Beg, turned his warriors against the Ghaznavids in Khorasan. He moved south and then west, conquering but not wasting the cities in his path. In 1055 the caliph in Baghdad gave Tughril Beg robes, gifts, and the title King of the East. Under Tughril Beg's successor, Malik Shah (1072-92), Iran enjoyed a cultural and scientific renaissance, largely attributed to his brilliant Iranian vizier, Nizam al Mulk. These leaders established the observatory where Umar (Omar) Khayyam did much of his experimentation for a new calendar, and they built religious schools in all the major towns. They brought Abu Hamid Ghazali, one of the greatest Islamic theologians, and other eminent scholars to the Seljuk capital at Baghdad and encouraged and supported their work.

A serious internal threat to the Seljuks, however, came from the Ismailis, a secret sect with headquarters at Alumut between Rasht and Tehran. They controlled the immediate area for more than 150 years and sporadically sent out adherents to strengthen their rule by murdering important officials. The word assassins, which was applied to these murderers, developed from a European corruption of the name applied to them in Syria, hashishiyya, because folklore had it that they smoked hashish before their missions.

Source taken from http://itsa.ucsf.edu/~ico/history/