Excerpts from Books and Wikipedia
Ali was Muhammad's cousin and son-in-law. Ali became caliph in 656, after the murder of Caliph Uthman. "He soon faced a rebellion by Uthman's cousin, Muawiya, who was outraged by Ali's refusal to punish his kinsman's murderers." Muslims (Sunnis) fought Muslims (Shi'ites) for the first time. Ali was killed in 661 leaving Muawiya to become caliph. [Furtado: 1001 Days] "Shi'ih (also Shi'ite): Partisan of Ali and of his descendents as the sole lawful 'Vicars of the Prophet.' The Shi'ahs reject the first three Caliphs, believing that the successorship in Islam belonged rightfully to Ali (first Imam and fourth Caliph) and to his descendents by divine right." [Gail: Baha'i Glossary, p. 47] "In that highly contested succession lie the origins of many of the major shapes and terms of Islam that are mostly unknown or puzzling to non-Muslims: Shiites and Sunnis, caliphs and emirs, Umayyads and Abbasids, all of these crucial internal divisions. . . . But Ali ruled a mere five years before his caliphate came to a bloody end with his assassination in 661." [Menocal: Ornament of the World, p. 18-9]