Abu Bekr, the first successor of the Prophet Mohammed, was head of the Moslem community from 632 to 634. He set about patching up the internal unrest between tribes. Then Omar, caliph (head of the Moslem community) from 634 to 644, initiated an explosive expansion of Islam. He seized Syria, then Jerusalem and finally Damascus in 638 after having defeated Heraclius. In 635, other Arab troops launched an assault on the Sassanian Empire, and crossed the Euphrates. The downfall of the empire was well underway when the Arab horsemen dealt the deathblow to the Sassanid dynasty and overran Persia first entering Ctesiphon in 637. Successive victories were to follow. They emerged victorious from the engagement at Nahavand in 642, which left the way open for them to enter the Iranian plateau. The conquest of Persia continued with the fall of Afghanistan (651) and then Transoxiana (674).  

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 Harun 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, and a large number of Iranian customs and traditions were rapidly adopted in Baghdad.

From the second half of the 9^th Century a period of decline began, and by the middle of the 10^th Century, the Abbassid caliphs at Baghdad had no real political control over Iran. The governors whom the caliphs had appointed to administer the frontier provinces displayed a tendency to establish virtually independent local dynasties. Some of these included the Tahirids of Khurasan (820 - 873), the Samanids of Khurasan and Transoxiana (819 - 1005) and their offshoot, the Ghaznavids of Khurasan, Afghanistan and northern India (977 - 1186). 

In 945 the Buwayids, a local dynasty from Gilan occupied Baghdad. During their 110 years of rule, the Buwayids seized all political power from the Abbassid caliphs.