The thirty-year regime of the first Caliphs (Abu Bakr, 632-634; 'Umar I, 634-644; 'Uthman, 644-656; and 'All, 656-661)2 was a period of wide conquest. This was facilitated by the extreme exhaustion of Sasanian Iran and Byzantium, in the aftermath of twenty-four years of warfare between them; and in the case of Iran a specific cause of Arab victory was the separatist temper of the local Dihqans or feudal chieftains, and a consequent weakness of central power in the person of the King of Kings (Shahinshah). The Arab conquest of the Byzantine provinces amounting to about two-thirds of the whole Empire, is no part of our theme, and we shall not advert to it except to remark that it took place almost simultaneously with that of the dominions of Sasanian Iran. Palestine, Syria, Upper Mesopotamia and Egypt were conquered between 634 and 642. The subjugation of Transcaucasia started in 640 but did not finish until the early eighth century. Asia Minor was repeatedly invaded, without being conquered properly. The frontier between the Byzantine Empire and the Caliphate was stabilized at the beginning of the eighth century so as to run along the Taurus mountains and the upper reaches of the Euphrates. The fate of Sasanian Iran was different. The Arabs, commanded by Muthanna b. Haritha, invaded Sasanian Mesopotamia early in 633, and in March took Hira, the former capital of the Lakhmites. Carrying the day in 'the chain fight',3 Muthanna crossed the Euphrates. In May of the same year Khalid b. al-Walid won a victory over the Persians near Ullays, and early next year his army was thrown into Syria (Source: Islam in Iran).
Persian Cultural Notion of Islam: Absolute Orientationism
