They view Husayn as the defender of Islam from annihilation at the hands of Yazid I.Husayn is the last imam following Ali whom all Shiah sub-branches mutually recognize. The Fatimid depiction of Ali's sword as carved on the Gates of Old Cairo, namely Bab al-Nasr shown below.

This dispute eventually led to the First Fitna, which was the first major civil war within the Islamic Caliphate.

The Fitna began as a series of revolts fought against Ali ibn Abi Talib, caused by the assassination of his political predecessor, Uthman ibn Affan.

While they were preparing his body, Abu Bakr, Umar, and Abu Ubaidah ibn al Jarrah met with the leaders of Medina and elected Abu Bakr as caliph.

Initially, Ali did not accept the caliphate of Abu Bakr and refused to pledge allegiance to him.

Shia Islam primarily contrasts with Sunni Islam, whose adherents believe that Muhammad did not appoint a successor and consider Abu Bakr (who was appointed Caliph through a Shura, i.e. Shia consider Ali to have been divinely appointed as the successor to Muhammad, and as the first Imam.

The Shia also extend this Imammah doctrine to Muhammad's family, the Ahl al-Bayt ("the people/family of the House" Although there are many Shia subsects, modern Shia Islam has been divided into three main groupings: Twelvers, Ismailis and Zaidis, with Twelver Shia being the largest and most influential group among Shia.

Did you not claim before the Ansar that you were more worthy than they of the caliphate because Muhammad came from among you (but Muhammad was never from Abu Bakr family) – and thus they gave you leadership and surrendered command?

I now contend against you with the same argument…It is we who are more worthy of the Messenger of God, living or dead.

This is indicated in both Sunni and Shia sahih and authentic Hadith.