The Khawarij (Arabic: الخوارج‎, al-Khawārij, singular خارجي, khāriji), Kharijites) were members of a school of thought that appeared in the first century of Islam during the First Fitna, the crisis of leadership after the death of Muhammad. It broke into revolt against the authority of the Caliph Ali after he agreed to arbitration with his rival, Muawiyah I, to decide the succession to the Caliphate following the Battle of Siffin (657). A Khariji later assassinated Ali, and for hundreds of years, the Khawarij were a source of insurrection against the Caliphate. The Khawarij opposed arbitration as a means to choose a new ruler on the grounds that "judgement belongs to God alone".