First caliph of Islam (632–34). He was one of the earliest converts to Islam and a close companion of the Prophet Muhammad, who married his daughter Aisha. When he succeeded to Muhammad’s position as temporal leader of the Muslim community, this pious and gentle man was chiefly concerned to reaffirm the allegiance of those Arabian tribes who had withdrawn it at the time of the Prophet’s death. These ‘wars of apostasy’ initiated the Arab conquests.