Answer
Mu'awiya opposed Ali after the assassination of Uthman. Mu'awiya was the governor of Syria and a member of the Umayyad clan. He believed that he should be the next caliph, rather than Ali, who was a member of the Hashemite clan. The dispute between the two clans led to the outbreak of the First Fitna, a civil war that lasted from 656 to 661.