After the assassination of the third caliph Uthman in 656 by provincial rebels, the caliphate fell into civil war as Mu'awiya ibn Abi Sufyan, a relative of Uthman and the governor of Syria, challenged the legitimacy of the new caliph Ali. The indecisive battle between the two at Siffin ended in an arbitration agreement in July 657. Asserting that human arbitration was invalid as God's command was clear that the rebels (in this case Mu'awiya) had to be fought and overcome, some of Ali's soldiers left the army. They were called Kharijites following this secession. They later fought against Ali in the Battle of Nahrawan in July 658 and were crushed by the Caliph. Following Ali's assassination in 661 by a Kharijite, Mu'awiya became the sole ruler, establishing the Umayyad Caliphate. During his reign, the Kharijites flourished in the southern Iraqi city of Basra. Severely repressed by the Iraqi governor Ziyad ibn Abih and later Ubayd Allah ibn Ziyad, the Kharijites fled to Arabia around 680.