Fitna intensified sectarian tendencies in Islam, and various doctrines were developed within what would later become the Sunni and Shi'a denominations of Islam. Background After the third caliph Uthman's assassination by rebels in 656, the rebels and the townspeople of Medina declared Ali, a cousin and son-in-law of the Islamic prophet Muhammad, caliph. Most of the Quraysh (the grouping of Meccan clans to which Muhammad and all the early caliphs belonged), led by Muhammad's prominent companions Talha ibn Ubayd Allah and Zubayr ibn al-Awwam, and Muhammad's widow A'isha, refused to recognize Ali. They called for revenge against Uthman's killers and the election of a new caliph through shura (consultation). These events precipitated the First Fitna. Ali emerged victorious against these early opponents at the Battle of the Camel near Basra in November 656, thereupon moving his capital to the Iraqi garrison town of Kufa. Mu'awiya, the governor of Syria, and a member of the Umayyad clan to which Uthman belonged, also denounced Ali's legitimacy as caliph, and the two confronted each other at the Battle of Siffin. The battle ended in a stalemate in July 657 when Ali's forces refused to fight in response to Mu'awiya's calls for arbitration. Ali reluctantly agreed to talks, but a faction of his forces, later called the Kharijites, broke away in protest, condemning his acceptance of arbitration as blasphemous. Arbitration could not settle the dispute between Mu'awiya and Ali. The latter was assassinated by a Kharijite in January 661, after Ali's forces had killed most of the Kharijites at the Battle of Nahrawan. Ali's eldest son Hasan became caliph, but Mu'awiya challenged his authority and invaded Iraq. In August, Hasan abdicated the caliphate to Mu'awiya in a peace treaty, thus ending the First Fitna. The capital was transferred to Damascus. Yazid's succession The treaty brought a temporary peace, but no framework of succession was established. As it had in the past, the issue of succession could potentially lead to problems in the future. The orientalist Bernard Lewis writes: "The only precedents available to Mu'āwiya from Islamic history were election and civil war. The former was unworkable; the latter had obvious drawbacks." Mu'awiya wanted to settle the issue in his lifetime by designating his son Yazid as his successor. In 676, he announced his nomination of Yazid. With no precedence in Islamic history, hereditary succession aroused opposition from different quarters and the nomination was considered