What happened to Sunnis and Shiites to separate them?

Answer 1

Originally, the two sects split over political disagreements over Succession after Mohammed's death. The Muslim community was united while Mohammed was the leader of this community. Most Muslims hold, however, that he never specifically chose a successor to his leadership. There was a minority in the community that supported the candidacy of 'Ali, the Prophet's son-in-law, this political faction became known as the "Supporters of 'Ali" which in Arabic is Shiat 'Ali (where the modern term "Shiite" comes from). They derived their support from specific hadiths and events that they claimed showed that God had revealed to Mohammed that 'Ali would succeed him. The majority of Muslims held that these hadiths and events showed nothing more than that 'Ali was very pious, something they did not deny. Therefore they gave power to the man who was Mohammed's second-in-command and father-in-law Abu Bakr. This majority were called the People of the Customs [of the Prophet] which in Arabic is Ahl Sunna (from where the modern term "Sunni" comes from.

Since this point, the religious separation of Sunnis and Shiites has resulted in additional religious differences. However, far more important than the theological differences, which are relatively minor, are the ethno-religious identities that have become attached to those labels. Religious discrimination of Sunnis against Shiites and religious discrimination of Shiites against Sunnis has been a staple of Sunni-Shiite relations.