The simplest explanation for the beginnings of Shiism is the disagreement concerning the succession of Muhammad, with the Shiat Ali ('Party of Ali', whence Shiites) supporting the Prophet's cousin and son-in-law Ali. Since Islam had emerged as both a religious and a political movement, conflicts about leadership also concerned both aspects, although in certain situations one was probably more pronounced or significant than the other. The difficulty of defining the religious and political dimensions of Shiism can be traced back to its origins and presents a persistent problem. Independent of this difficulty, the events between the death of Muhammad in 632 and that of his grandson Husayn in 680 were crucial for the formation of Shiism as a political force, and equally critical for key features of its religious outlook, in particular its salvation history.
Due to the lack of evidence and the bias of later sources, it remains unclear whether the Prophet Muhammad, the founder of Islam, had chosen a successor or established principles of determining one. While Shiite tradition maintains that he appointed Ali at Ghadir Khumm, Sunnis claim that Muhammad did not choose a successor. Events in the first decades of Islamic history suggest that even if Muhammad had expressed views on these matters, disagreement prevailed among his followers. Those who decided to follow another prophet remained a minority and were defeated in the ridda ('apostasy') wars (632-633).
There was also a broad consensus (the below-mentioned Kharijites being the only exception) that Muhammad's successor should belong to the same tribe, the Quraysh. The main forces in the young Muslim community were the early Meccan converts to Islam, their helpers (Arabic: ansar) in Medina, and the old Meccan elites who accepted Islam only following military defeat. Apart from leadership, the conquests and the shape of the Islamic polity were among the main problems to negotiate.
When Muhammad died, a public gathering took place in the course of which Abu Bakr, the Prophet's father-in-law and close companion, was elected on the spot and by public acclamation as the first caliph. Abu Bakr appointed his successor, Umar, who succeeded him upon his death in 634. While the supporters of Ali disagreed with the choice of the first and the second caliphs, the conflict escalated when Uthman became the third caliph in 644.
According to later historiography, Uthman was a controversial candidate because of his ancestry (among other reasons). He belonged to the descendants of Abd Shams, who had been the leading group in Mecca. Some of them had led the opposition against Muhammad. Abu Sufyan, the father of the future Umayyad caliph Muawiya, was one of their leaders. Abu Sufyan and his sons waited until the last moment before they converted to Islam. Furthermore, Uthman was accused of not taking religious principles very seriously and of giving benefits to his Umayyad relatives instead of privileging, as Umar had done, the early converts. Another controversial decision was to centralize the growing Islamic Empire.
In 656 Uthman was murdered by a group from Fustat, ancient Cairo. The event was traumatic for many reasons, but especially because he was killed while reading the Quran. Ali used the opportunity and had himself appointed as the next caliph, but soon faced resistance. Umayyad opposition was forming in Syria. Furthermore, Muhammad's widow Aisha and her two allies, Zubayr and Talha (both close companions of the Prophet), challenged Ali in the Battle of the Camel outside Basra in December 656. Ali emerged victorious from this confrontation, guaranteeing Aisha a place on the Shiites' list of villains. In the Shiite tradition, the unjust behavior of Ali's and later his son Husayn's enemies provided a blueprint for describing their later opponents.
The community divided over the rightfulness of Uthman's murder. Ali, who protected the murderers, was entrenched in the two garrison cities on the Euphrates: Kufa, which had rebelled against Uthman, and Basra. The Umayyad family with their leader Muawiya in Syria did not acknowledge Ali's election and demanded revenge for the third caliph's death. The armies of Ali and Muawiya met in 657 in the Battle of Siffin, and when Ali accepted arbitration with Muawiya, some of Ali's followers left in protest and formed the first clearly distinct sect within Islam, the Kharijites. A member of this community assassinated Ali in 661, and Muawiya assumed the caliphate as the first of the Syrian Umayyads. The period between 656 and 661 is known as the First Civil War or First Fitna in Islamic history. For the early Shiites, this was the only time since Muhammad's death that the right person has led the Muslim community.
In these early days, one's stance on succession probably depended on the individual candidates, but also on more general principles, i.e., whether or to what extent piety (as expressed in pious deeds such as early conversion) outweighed close genealogical ties with Muhammad. Furthermore, in all likelihood, support and opposition for leaders also depended on the benefits that people could expect from them. While the specific political strategies realized by individual Sunni or Shiite rulers continued to determine to what extent they were supported by members of their own as well as other branches of Islam, the general principles of legitimacy of rule and the importance of being related to Ali in particular became crucial to sectarian loyalties and the political language of Shiism.