The Battle of Siffin was fought under the Caliphate of ‘Ali. After the assassination of the third caliph, ‘Uthmān, ‘Ali was elected as the fourth caliph. However, his caliphate was disputed by Mu‘āwiya, the governor of Syria, who refused to pledge allegiance to him. This led to a series of battles including the Battle of Siffin, which was fought between ‘Ali's forces and Mu‘āwiya's forces in the year 657 CE. The battle ended in a stalemate, and a peace agreement was eventually reached.