Although Ali was able to replace Uthman's governors in Egypt and Iraq with relative ease, Mu'awiya had developed a solid power-base and an effective military against the Byzantines from the Arab tribes of Syria. Mu'awiya did not claim the caliphate, but was determined to retain control of Syria and opposed Ali in the name of avenging his kinsman Uthman, accusing the caliph of culpability in his death. Ali and Mu'awiya fought to a stalemate at the Battle of Siffin in early 657. Ali agreed to settle the matter with Mu'awiya by arbitration, though the talks failed to achieve a resolution. The decision to arbitrate fundamentally weakened Ali's political position as he was forced to negotiate with Mu'awiya on equal terms, while it drove a significant number of his supporters, who became known as the Kharijites, to revolt. Ali's coalition steadily disintegrated and many Iraqi tribal nobles secretly defected to Mu'awiya, while the latter's ally Amr ibn al-As ousted Ali's governor from Egypt in July 658. In July 660 Mu'awiya was formally recognized as caliph in Jerusalem by his Syrian tribal allies. Ali was assassinated by a Kharijite in January 661. His son Hasan succeeded him, but abdicated in return for compensation upon Mu'awiya's arrival to Iraq with his Syrian army in the summer. At that point, Mu'awiya entered Kufa and received the allegiance of the Iraqis.