Answer
Mu'awiya I, the governor of Syria, opposed Hasan, the fourth caliph of the Rashidun Caliphate. Hasan was forced to abdicate in favor of Mu'awiya, who became the first caliph of the Umayyad dynasty. This marked a significant turning point in Islamic history, as the Umayyads established a centralized empire that ruled over a vast territory for several centuries.