Saylor.org's Ancient Civilizations of the World/The Arab Empire of the Umayyads
The Arab Empire of the Ummayyads: Caliphate and civil war (632–750) 
With Muhammad's death in 632, disagreement broke out over who would succeed him as leader of the Muslim community. Umar ibn al-Khattab, a prominent companion of Muhammad, nominated Abu Bakr, who was Muhammad's companion and close friend. Others added their support and Abu Bakr was made the first caliph. Abu Bakr's immediate task was to avenge a recent defeat by Byzantine forces, although he first had to put down a rebellion by Arab tribes in an episode known as the Ridda wars, or "Wars of Apostasy".
His death in 634 resulted in the succession of Umar as the caliph, followed by Uthman ibn al-Affan, Ali ibn Abi Talib and Hasan ibn Ali. The first caliphs are known as al-khulafā' ar-rāshidūn ("Rightly Guided Caliphs"). Under them, the territory under Muslim rule expanded deeply into Persian and Byzantine territories. When Umar was assassinated in 644, the election of Uthman as successor was met with increasing opposition. In 656, Uthman was also killed, and Ali assumed the position of caliph. After fighting off opposition in the first civil war (the "First Fitna"), Ali was assassinated by Kharijites in 661. Following this, Mu'awiyah seized power and began the Umayyad dynasty, with its capital in Damascus.
These disputes over religious and political leadership would give rise to schism in the Muslim community. The majority accepted the legitimacy of the three rulers prior to Ali, and became known as Sunnis. A minority disagreed, and believed that Ali was the only rightful successor; they became known as the Shi'a. After Mu'awiyah's death in 680, conflict over succession broke out again in a civil war known as the "Second Fitna". The Umayyad dynasty conquered the Maghrib, the Iberian Peninsula, Narbonnese Gaul and Sindh. The local population of Jews and indigenous Christians, persecuted as religious minorities and taxed heavily, often aided Muslims to take over their lands from the Byzantines and Persians, resulting in exceptionally speedy conquests.
The Umayyad aristocracy viewed Islam as a religion for Arabs only; the economy of the Umayyad empire was based on the assumption that a majority of non-Muslims (Dhimmis) would pay taxes to the minority of Muslim Arabs. A non-Arab who wanted to convert to Islam was supposed to first become a client of an Arab tribe. Even after conversion, these new Muslims (mawali) did not achieve social and economic equality with the Arabs. The descendants of Muhammad's uncle Abbas ibn Abd al-Muttalib rallied discontented mawali, poor Arabs, and some Shi'a against the Umayyads and overthrew them with the help of the general Abu Muslim, inaugurating the Abbasid dynasty in 750 and moved the capital to Baghdad.