When Umar was assassinated by Persians 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. This led to the first civil war (the "First Fitna") over who should be caliph. The Kharijites have been one of the main contenders within the war. They hold the sins of the caliphs put them outside of Islam and called for killing them as apostates. Unlike for the later Sunnis and Shia, a caliph requires piety only and does not need to be a descendant of Muhammad, his family or his tribe. The Kharijites assassinated Ali in 661. To avoid further fighting, the new caliph Hasan ibn Ali signed a peace treaty, abdicating to Mu'awiyah, beginning the Umayyad dynasty, in return that he not name his own successor. These disputes over religious and political leadership would give rise to a schism in the Muslim community. The majority accepted the legitimacy of the first four leaders and became known as Sunnis. A minority disagreed and believed that only Ali and some of his descendants should rule; they became known as the Shia. The Kharijites failed to nominate their caliph as aspirants were easily accused of sin and regarded as apostates. Mu'awiyah appointed his son, Yazid I, as successor and after Mu'awiyah's death in 680, the "second civil war" broke out, where Husayn ibn Ali was killed at the Battle of Karbala, a significant event in Shia Islam. Sunni Islam and Shia Islam differ in some respects.