First Umayyad caliph and nephew of the third Sunni caliph, Uthman. Opposed Ali as caliph because he did not track down Uthman's murderer. This opposition led the Kharijis to break away from him since he had challenged the authority of a caliph they considered to be legitimate. He forced Hasan ibn Ali to abdicate the caliphate in his favor, and then moved the caliphate from Arabia to Damascus, symbolizing the transition to a new imperial age. The caliphate became an absolute monarchy under his rule.