The Sunni and Shi’a schism began as a dispute over political succession and eventually evolved into a theological one as well, not unlike the schism of 1054 between the Eastern and Western Churches.
Shi’as maintained that the right of succession for the leadership of the nascent Muslim community after the death of the Prophet Muhammad went to Ali, the cousin and son in-law of Muhammad, while Sunnis believed that the choice of Abu Bakr, father in-law and close confidant of Muhammad, and of the subsequent two caliphs or rulers was valid. When Ali was finally chosen as the fourth caliph or ruler, his rule was short-lived, and after his death his rival Muawiyyah quickly asserted his power and established Umayyad rule.
Many practices of the Umayyad dynasty, which had adopted a pattern of rule and succession that was starkly at odds with that of the Prophet Muhammad and the early caliphs, disturbed many Muslims, which led to a number of revolts by various groups. One of these revolts was led by Hussein, Ali’s son and the grandson of the Prophet Muhammad. When Hussein, who is revered by both Shi’as and Sunnis alike, was brutally killed along with many of his family members by the Umayyads at Karbala in Iraq, this crystallized the belief among supporters of Ali (Alids) that governance should have remained with the descendants of the Prophet Muhammad. Subsequent attempts to overthrow the Umayyads by another son of Ali and by others such as Abd ‘Allah ibn al-Zubayr, a grandson of the first caliph Abu Bakr, were unsuccessful until the Abbasid revolution in 750. While the Alids had supported the Abbasids, another branch of the Prophet’s family, believing that they would turn over rule to the Alid line, they were soon disappointed when the Abbasids claimed rule for themselves. In response, the Alids fomented a number of small and unsuccessful rebellions. Under increasing repression by the Abbasids, their political movement took on a more theological character.
The term “Shi’at Ali,” or “the faction of Ali,” at some point became merely Shi’a while the term “Sunni” came to include those who agreed upon the validity of the rule of all of the first four caliphs. While today there are theological differences between these two major Islamic sects, they are in agreement on the cardinal points of faith and practice.