Mu'awiya I
Mu'awiya I was the founder and first caliph of the Umayyad Caliphate. Previously, he served as the governor of Syria under the caliph Umar ()  and then his cousin and caliph Uthman (). After Uthman was murdered, Mu'awiya demanded Uthman's killers to be punished, but Ali () refused. Mu'awiya subsequently went to war with Ali for a few years. Ali was killed by the Khawarij in January 661 and Hasan was elected caliph. Hasan gave the caliphate to Mu'awiya and both made peace. During Mu'awiya's reign, the Arab empire conquered much of North Africa, as well as Khurasan and Sijistan. Constantinople, the capital of the Roman Byzantine empire, was also besieged. During the last years of his caliphate, Mu'awiya declared his son Yazid to be his successor; a move which was opposed by several Muslim leaders, including Husayn ibn Ali.