The third decade of the seventh century was a turning point in Iranian history, in which the pattern of the country's religious, cultural and psychological development was determined up to the present age. For anyone wanting an insight into modern Iran, the events of this period are extremely important, immensely exciting, and still rather mysterious. They were certainly totally unexpected; in 614 when Khosrow Parviz had a twenty-year career of successful conquest behind him, no one could possibly have foreseen that within twenty-five years not merely his dynasty but the whole fabric of Iranian life would have been engulfed and overwhelmed. After centuries of relative immobility, events moved with startling rapidity. It was not until 614 that Mohammad claimed to be a divinely inspired prophet. For eight years after that he was on exile from his native Arabia. He died in 632 within two years of entering Mecca. The Arab Conquests started only after his death, with an attack on Mesopotamia in 633. Yazdgird III, the last of Sassanian kings, was invited to embrace Islam. He contemptuously refused, pouring scorn on the Arabs for eating lizards and the practice of infanticide.
The invading Arabs succeeded in taking Ctesiphon in 637, and inflicting a crushing defeat on the Iranians at the Battle of Nahavand in 642. This brought to an end the last national Iranian dynasty for nearly a thousand years. The Arab Conquest permeated far deeper into the structure of Iranian civilization than any other before or since. It provided the country with anew religion and anew script; it influenced its language and revolutionized its art. Yet it did not destroy utterly or absorb completely; what was indigenous in Iranian character and customs was driven underground and emerged in new and complex forms. The rise of Islam as a religion replacing Zoroastrianism is one of the greatest events in world history.
 

Various reasons can be adduced for the success of this invasion: It was more spiritual than material; the birth of a crusading religion in Arabia coincided with the exhaustion of a dynasty in Iran; Islam was democratic while Zoroastrianism was exclusive and feudal; four centuries of independence under autocratic rule had sapped initiative and reduced the will to resist. But none of these considerations fully explain the completeness with which Iran apparently succumbed to Islam. Islam soon became the dominant religion in Iran, as a result of which the subsequent revolts lost their religious character and turned into political turmoil.
The broad sweep of history of this period is fascinating, but its details are tedious in the extreme. It is enough to say here that, from the middle of the ninth century onwards, the power of the Abbasid Caliphate steadily declined. In Iran there sprang up a number of small semi-independent dynasties, some of which are just worth mentioning by name. There were, for example, the Saffarids, or coppersmiths, founded by a highway robber and based on Sistan; and Samanids, mainly centered on modern Afghanistan. The Ghaznavids, who spread from Afghanistan to India and also made various incursions into Persia In the early eleventh century were rather more important since they maintained themselves in power locally for over two centuries and had left substantial architectural remains in Afghanistan.