The killing of the third caliph Uthman in 656 and the civil war between the fourth caliph Ali and the governor of Syria, Mu'awiya (the First Fitna, 656–661), broke the political unity of the early Muslim community for the first time. After Ali was assassinated by a Kharijite in Kufa in 661, Mu'awiya was proclaimed caliph in Damascus. The move of the capital from Medina to Kufa and then to Damascus — an old Roman-Byzantine city — signalled that Arab-Muslim rule would now be exercised not by a coalition of desert tribes but by a state with a settled bureaucracy.

Mu'awiya's most contested decision was, before his death in 680, to designate his son Yazid as heir, effectively turning the caliphate into a dynastic, father-to-son succession. This is among the foundations of the later Sunni-Shia split; Husayn ibn Ali's refusal to pledge allegiance to Yazid was the direct cause of his march toward Kufa and his death at Karbala in 680. The dynasty nevertheless ruled, from Damascus, the eastern Mediterranean coast, Egypt, North Africa, the Iranian plateau and parts of Inner Asia for ninety years.

The geography of conquest expanded extraordinarily across the Umayyad century. In 711 Tariq ibn Ziyad crossed the Strait of Gibraltar and overran Visigothic Spain within a few years; in the same year Muhammad ibn al-Qasim took Sind (in today's Pakistan); in 717–718 Constantinople was placed under a great siege, but the Byzantine walls, Greek fire and a hard winter ended the attempt in failure. More durable were the reforms under Caliph Abd al-Malik ibn Marwan (685–705): the administrative registers were translated from Greek and Pahlavi into Arabic, the first fully Arabic-script gold dinar was struck in 696/697, and the Dome of the Rock was raised in Jerusalem (691/692).

The real legacy of the Umayyad century lies less in conquest than in this infrastructure: a single administrative language (Arabic), a single coinage (dinar / dirham) and a shared legal-religious frame of reference stretching from the Atlantic to Sind. The dynasty would fall in the Abbasid revolution of 750; a single Umayyad prince — Abd al-Rahman al-Dakhil — would escape and found a separate Umayyad emirate in al-Andalus. The multi-ethnic Mediterranean-to-Asian state model the Umayyads built nevertheless shaped the administrative memory of the Muslim world for the next thousand years.