Shi'a Muslims trace the division to the death of the Prophet Muhammed, when Abu Bakr was chosen as caliph rather than Ali In the Shi'a view, Ali and his followers had a religious basis for their position that the caliph, or successor, must come from the Prophet's family.
Sunni Muslims trace the division to the killing of Hussein, the grandson of the Prophet Muhammed, along with his family in Karbala, Iraq, by one of the Umayyad Caliph Yazid's generals, fifty years after the death of the Prophet Muhammed. The people of Iraq regretted their failure to support Hussein which resulted in his death. Subsequently they began a political movement to overthrow the Umayyads, who were not only responsible for his death, but had become corrupt and dynastic rulers.>br?
Attempts to overthrow the Umayyads were unsuccessful until the Abbasid revolution in 750 C.E. After the Abbasids came to power, however, the people who supported rule by the descendants of Hussein were increasingly suppressed. Sunnis believe that this political dispute then took on a more theological nature.