It is ironic that a man who was to become the political-religious head of Islam was born (c. 602) into a clan ('Abd Shams) that rejected the Prophet Muhammad in his home city, Mecca, and continued to oppose him on the battlefield after he had emigrated to Medina. Mu'awiyah's father was Abu Sufiyan ibn Harb who was a bitter opponent of Muhammad.
Mu'awiyah did not become a Muslim until Muhammad had conquered Mecca and had reconciled his former enemies by gifts. Possibly as a part of Muhammad's policy of conciliation, Mu'awiyah was made a scribe in his service. But Mu'awiyah's contributions to Islamic history are wholly associated with his career in Syria, which began shortly after the death of the Prophet, when he, along with his brother Yazid, served in the tribal armies sent from Arabia against the Byzantine forces in Syria.
Upon the death of Yazid in 640, Mu'awiyah was appointed governor of Damascus by the caliph 'Umar and gradually gained mastery over other areas of Syria. By 647 Mu'awiyah had built a Syrian tribal army strong enough to repel a Byzantine attack and in subsequent years to take the offensive against the Byzantines in campaigns that resulted in the capture of Cyprus (649) and Rhodes (654) and a devastating defeat of the Byzantine navy off the coast of Lycia in Anatolia (655). At the same time, Mu'awiyah periodically dispatched land expeditions into Anatolia. All these campaigns, however, came to a halt with the accession of Ali ibn Abi Talib to the caliphate, when a new and decisive phase of Mu'awiyah's career began.
As a kinsman of the slain caliph 'Uthman, Mu'awiyah bore the duty of revenge. Because 'Ali neglected to apprehend and punish 'Uthman's murderers, Mu'awiyah regarded him as an accomplice to the murder and refused to acknowledge his caliphate. Thereupon 'Ali marched to the Euphrates border of Syria and engaged Mu'awiyah's troops at the famous Battle of Siffin (657). Mu'awiyah's guile turned near defeat into a truce. Resorting to a trick that played upon the religious sensibilities of 'Ali's forces, he persuaded the enemy to enter into negotiations that ultimately cast doubt on the legitimacy of 'Ali's caliphate and alienated a sizable number of his supporters. When these former supporters--the Kharijites--rose in rebellion against 'Ali, Mu'awiyah took advantage of 'Ali's difficulties in Iraq to send a force to seize control of Egypt. Thus, when 'Ali was assassinated in 661, Mu'awiyah held both Syria and Egypt and, as commander of the largest force in the Muslim Empire, had the strongest claim to the caliphate. 'Ali's son Hasan was persuaded to remove himself from public life in exchange for a subsidy, which Mu'awiyah provided.