The Umayyad and Abbassid Caliphates

After the death of the Prophet in 632, the direction of the Islamic world was handed over to the caliphs (from the Arab word khalifa, successor or replacement). The first four caliphs, Abu Bakr, Omar, Osman and Ali, were elected by the Muslim community. But  the assassination of Osman and the election of Ali as successor created a political crisis which ended in the replacement of Ali by his rival Moawiya. The latter transferred the capital from Medina to Damascus and set up a hereditary caliphate, known as the Umayyad Caliphate (661-750). This move aroused the anger of two groups: the inhabitants of Medina, who favoured an electoral system for the nomination of the caliphs; and the Shi'ites, for whom the leadership of the community should be handed over to the descendants of Ali .
In 633, shortly after the Prophet's death, the first caliph Abu Bakr sent an Arab army to conquer Iraq. The last Sassanian ruler, Yazdgard Ht (632-651), was powerless to prevent its advance; in 637, the Persian army was defeated at Al-Qadisiyah and the Sassanian capital, Ctesiphon, fell. In 642, Sassanian power was destroyed once and for all at the battle of Nebvand, near Marnen, and Yazdgard fled to Merv, where he was assassinated in 651.
The conquest of Persia continued during the rule of the first Umayyad caliphs; in a very short time, Afghanistan (651) and then Transoxiana (674) also fell. But opposition to the caliphate was growing and a series of rebellions and riots broke out, one of which, supported by the Shi'ites, led to the restoration as caliph in 750 of Abul Abbas, a descendant of the Prophet.
The Abbasid dynasty (750-945) established its capital at Baghad, near the old Sassanian capital. For a century, the empire experienced a time of unprecedented cultural, artistic and economic development, particularly during the reigns of Hrun al-Rashid (786-809) and al-Mamun (813-833). Persian scholars and artists played an important role in this intellectual activity: from the very beginning of the Abbasid Caliphate , they had been placed in charge of the highest court functions to which they had not had access in Damascus, and a large number of Iranian customs and traditions were rapidly adopted in Baghdad.
From the second half of the ninth century, a period of decline began which was marked by the growing power of the Turkish and Slav mercenaries who had been the caliph guards. In 908, the head of the guard was named Emir of Emir by the thirteen-year-old caliph, and thus became the de facto ruler of the empire. The governors of the Iranian provinces took advantage of this period of anarchy to break away from Baghdad, forming small local dynasties. In the east, in Khorassn, these were the Tahirid (821-873), Saffarid (867-963) and Samanid (892-999) dynasties. The Samanids, whose capita! was at Bukhara, favoured Persian culture and literature, introducing Persian as the written language of the bureaucracy.
In the west, there was a general movement of people from the mountains near the Caspian Sea towards the plateau, first the Ziyarids (928-1077), then the Buyids (945-1 055) , a Shi'ite dynasty that took control of Khuzestan, Shiraz, Iraq and finally Kerman. Tn 945, the Buyid prince Mu'izz al-Daula entered Baghdad and the Abbasid caliph promptly named him Emir of Emirs.

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