In 661 CE, the Prophet Muhammad's cousin and son-in-law, Ali ibn Abi Talib, became the fourth Caliph of Islam. However, his reign was marked by conflicts with the Umayyad clan, who sought to restore their dominance. After Ali's assassination in 661 CE, his son Hasan ibn Ali succeeded him as Caliph but was forced to abdicate in favor of the Umayyad leader, Muawiya ibn Abu Sufyan. Upon Muawiya's death in 680 CE, his son Yazid I succeeded him, and the Umayyad dynasty consolidated its power.