Upon the death of the Prophet
, a dispute emerged in the Islam
. Those who believed succession should fall to the Prophet's companions, known as the Caliph
s, were called Sunni
. Those who believed in following the Prophet's bloodline, beginning with Muhammad's cousin and son-in-law Ali
, were known as Shia
When the Caliph Uthman was assassinated, Ali was chosen to succeed him, but the Umayyad clan contested this choice, and the dispute became a schism. When Ali died, his sons Hassan and Hussein resumed the struggle. Hassan was bought off, but in 680 Hussein led the rebels against an Umayyad army led by Omar ibn Sad at Karbala, where they were defeated and Hussein martyred.
Because of the fratricidal nature of the contest, and in particular because of the defection on moral grounds of an Umayyad subcommander named Horr and the pitched debate he had with Omar just before the battle, Shias see Karbala as a pivotal battle in the struggle between Good and Evil, with the part of Satan taken by Omar.