In 657 CE the caliph Ali ibn Abi Talib faced the Umayyad governor of Syria, Mu'awiya, at the Battle of Siffin, fought over responsibility for the murder of the previous caliph, Uthman. As Ali's forces gained the upper hand, Mu'awiya's side raised pages of the Quran on their spears, calling for the dispute to be settled by arbitration rather than continued combat, and Ali, under pressure from part of his own army, agreed. A substantial faction within his camp regarded this as a betrayal of principle, arguing that judgement belonged to God alone, expressed in the slogan la hukma illa lillah, no judgement but God's, and that submitting the caliphate to human arbitration was itself an act of unbelief. This faction withdrew from Ali's army, earning the name Kharijites, those who go out, and encamped separately, from where they began raiding and denouncing both Ali and Mu'awiya as illegitimate.
Ali marched against the main Kharijite force and crushed it at the Battle of Nahrawan in 658 CE, but the movement was not eliminated. A surviving Kharijite, Ibn Muljam, assassinated Ali in Kufa in 661 CE, striking him with a poisoned sword while he prayed, in direct reprisal for Nahrawan, an act that left a permanent mark on Sunni and Shia memory of the first Islamic civil war alike.