Further information: Succession to Muhammad, Muslim conquests, and Arab Empire
Muhammad began preaching Islam at Mecca before migrating to Medina, from where he united the tribes of Arabia into a singular Arab Muslim religious polity. With Muhammad's death in 632, disagreement broke out over who would succeed him as leader of the Muslim community. Umar ibn al-Khattab, a prominent companion of Muhammad, nominated Abu Bakr, who was Muhammad's intimate friend and collaborator. Others added their support and Abu Bakr was made the first caliph. This choice was disputed by some of Muhammad's companions, who held that Ali ibn Abi Talib, his cousin and son-in-law, had been designated his successor. Abu Bakr's immediate task was to avenge a recent defeat by Byzantine (or Eastern Roman Empire) forces, although he first had to put down a rebellion by Arab tribes in an episode known as the Ridda wars, or "Wars of Apostasy".[59]
The territory of the Caliphate in the year 750His death in 634 resulted in the succession of Umar as the caliph, followed by Uthman ibn al-Affan and Ali ibn Abi Talib. These four are known as al-khulaf
