In 632 CE the Prophet Mohammad died, and his succession became disputed. His first successor was his father in law, Abu Bakr, even though Mohammad’s son in law, Ali ibn Abi Talib claimed Mohammed passed leadership of the caliphate (Islamic State) directly to him. In any case, in a show of solidarity for the small Muslim community, Ali accepted Abu Bakr’s leadership.
Over the next decades, Abu Bakr and his two successors were assassinated during early infighting among the Arabic tribes and Ali became the fourth caliph. Ali’s followers claimed that this was divine will in action because Ali was the only caliph directly chosen by Mohammad. Ali spread the Caliphate from the Arabian Peninsula north to the Caucus Mountains, east across Persia, and west across the north African coast. Ali was assassinated in Kufa (modern Najaf, Iraq) in 661.
Ali’s followers declared his son Hussein Ali the caliph. Hussein Ali was Mohammad’s grandson by blood through his mother, Mohammed’s daughter. But his followers were suppressed by Muawiyah, the son of the assassinated third caliph, who had a larger army and power base in the Levant and Syria. Muawiyah became the first caliph of the Umayyad Caliphate. But Ali’s supporters did not recognize him because he lacked a blood tie to Mohammad, and fought a low level insurgency in support of Hussein Ali for caliph. Twenty years later in 680, Muawiyah died, and his son Yazid was nominated caliph. In response, Kufa and most of Mesopotamia openly declared for Ali and in rebellion against Umayyad Caliphate. Unfortunately for Ali, he was in Mecca at the time, and this put him in a precarious position, so he raced back to his power base in Kufa with a thousand followers.
On 10 October 680 CE Ali got as far as the Karbala Pass, which was blocked by a vastly superior army led by Yazid. In the ensuing battle, Ali, his family, and all of his followers were massacred. Ali’s followers never forgave Yazid. This led directly to the schism of Islam into the Sunni and Shia branches. Hussein Ali was/is considered a martyr by his followers and they would accept only his descendants as leaders of the caliphate. They would go on to form the Shia branch of Islam when Ali’s followers subsumed Persia, and Islam was influenced by the early Persian Zoroastrianism. Shia’s leadership by bloodline formed a more centralized and hierarchical Islam as seen in Iran with the rule by the Ayatollahs today (all of whom claim to be descendants of Ali, and hence Mohammad). Yazid and his successors went on to form the Sunni, and without a direct blood tie to Mohammad, a much more decentralized branch of Islam. The Shia consider Yazid, all of his successors, and the Sunni in general as usurpers, and this is the basis of the Sunni/Shia divide in Islam today.