By 735 an Arab empire stretched from Arles and Avignon in southern France to the Indus River and Central Asia, and a vital young civilization fostered by a new world religion was taking root. Yet the Muslim conquerors were divided by tribal quarrels, tensions among new converts, and religious revolts. In 745 a vigorous new successor to the Prophet took control in Damascus and began to restore the waning power of the Umayyad dynasty. Marwān II's...