Muawiya, governor of Syria and Ali's opponent at Siffin, took the caliphate in 661 and moved its capital to Damascus. His decisive innovation was to name his son as successor, converting an office that had been conferred by acclamation into a dynasty, and it was that decision that produced the event on which Shia Islam turns. In 680 Husayn, the Prophet's grandson, refused allegiance to the new caliph Yazid and was intercepted with his small following on the plain of Karbala in Iraq, where he and most of his household were killed. The mourning of that day, Ashura, became the central devotional act of Shia Islam and the model of righteous suffering against unjust power.
Abd al-Malik, who reigned from 685 and put down a second civil war, remade the empire's public face. He made Arabic the language of administration in place of Greek and Persian, replaced the borrowed Byzantine and Sasanian coin types with a purely epigraphic Islamic coinage carrying Quranic text and no image, and in 691 or 692 completed the Dome of the Rock in Jerusalem.
That building is the earliest surviving monument of Islamic architecture, and its inscriptions, which affirm God's oneness and address Christians directly on the status of Jesus, are the first public statement of Islam as a religion distinct from those around it rather than a movement inside them.