Mu'āwiya reigned as universally recognized caliph from 661 to 680. His power rested chiefly on the army composed of the Arabs settled in Syria, and he made Damascus his capital. In the practice of the nomadic Arabs a chief was usually succeeded by the best qualified member of his family; primogeniture and even sonship gave no special rights. This gave little guidance in arranging for the succession to the caliphate. Mu'āwiya tried to have his son Yazīd acknowledged as successor before his own death, but even so there were some who did not accept Yazīd. The opposition led to a catastrophic civil war when Yazīd died in 683, leaving only a minor son. 'Abd-Allāh ibn-az-Zubayr (or, more simply, Ibn-az-Zubayr), who had defied Yazīd from Mecca, now gained control of much of Iraq as well as of the region of Mecca and Medina. There was widespread confusion, and vast tracts of the caliphate were under the effective control of neither the Umayyads nor Ibn-az-Zubayr. Under the leadership of a member of another branch of the family the Umayyads fought back; in 691 they completed the recovery of Iraq, and before the end of 692 extinguished the last flames of revolt in Mecca.

The expansion of the caliphate, which had continued under Mu'āwiya but had been stopped by the civil war, was now resumed. In the east the Muslims extended their sway to Central Asia and north-west India; while in north Africa they pressed westwards into Morocco, and in 711 crossed the straits into Spain. To the north there were frequent expeditions against the Byzantines, but no permanent occupation of territory proved possible. The vastness of the territories ruled led to ever-increasing internal tensions, and the clumsy administrative machine lumbered along with creaks and groans. From about 730 or 735 it must have been clear to acute observers that the empire was slowly breaking up, and some of these observers attempted, by staging a revolt, to create an alternative government.