Muawiyah, a kinsman of Uthman and the governor of Syria, now refuses to recognize Ali, and he demands the right to avenge his relative's death.

In what is perhaps the most important battle fought between Muslims, Ali's forces meet Muawiyah's at the Plain of Siffin near the largest bend of the Euphrates River.

Muawiyah's forces, seeing that they are losing, propose arbitration.

Accordingly, two arbitrators are chosen to decide whether Uthman's death had been deserved.

Such a decision would give his slayer status as an executioner rather than as a murderer and would remove the claims of Uthman's relatives.

When the arbitrators decide against Ali, he protests that the verdict is not in accordance with sharia (Islamic law) and declares his intention to resume the battle.