Ali, after defeating the revolt of 656, immediately faces a new revolt fomented by the champions of the Umayyad cause, led by Uthman's cousin Muawiyah, governor of Syria, who believes Ali was involved in the murder of Uthman.

Ali responds by invading Syria.

The two armies meet in May 657 along the Euphrates River at Siffin (near present Ar-Raqqah, Syria) where they engage in an indecisive succession of skirmishes, truces, and battles, culminating in July with the legendary appearance of Mawiyah's troops with copies of the Qur'an impaled on their lances—supposedly a sign to let God's word decide the conflict.

In fact, this second rebellion is on the point of being crushed when Muawiyah averts defeat by proposing arbitration.

'Ali sees through the stratagem but is forced by his army to accept adjudication by umpires, which greatly weakens his position.

'Ali delegates Abu-Musa al-Ashaari as his representative, while Muawiyah sends 'Amr ibn al-'As.

The two men, basing their conversations on the Qur'an and the traditions (Hadith) of the Prophet and in the presence of witnesses, are to decide whether Uthman had been guilty of abusing the divine law.

If he had sinned and his murder was justified, then Ali's position as caliph will be secure; a verdict of innocence, however, will justify Muawiyah's attempts at vengeance and dislodge Ali.