The ʿAbbasid period opened with a major political revolution in the Islamic world. The ʿAbbasid movement had developed in Khurāsān, the vast province which lay on the north-east frontiers of Islamic Iran, during the first part of the second/eighth century. The reasons for the revolt against the rule of the Umayyads in distant Damascus have been intensively debated by historians, and much remains unclear; but we can be certain that it was a movement among all the Muslims of the area, Arab and non-Arab alike, and it was intended to replace the Umayyad government, thought to be authoritarian and indifferent to both religion and the local concerns of the Khurāsānīs, by the rule of a member of the “Family of the Prophet” who would usher in an era of peace and justice. Perhaps because they came from a frontier province and had ample military experience, the Khurāsānīs were able to succeed where so many before them had failed; marching westwards across the great plains of central Iran and through the passes of the Zagros mountains, they took Iraq in 132/749 and, while the leaders stayed in Iraq and Iran to consolidate their position, an expedition was sent to the west to defeat the demoralized Umayyad army and eventually to kill the last Umayyad caliph, Marwan b. Muhammad, in Egypt, where he had taken refuge.