Following Muhammad's death in 632, Abu Bakr became leader of the Muslims as the first Caliph. After putting down a rebellion by the Arab tribes (known as the Ridda wars, or Wars of Apostasy), Abu Bakr attacked the Byzantine Empire. On his death in 634, he was succeeded by Umar as caliph, followed by Uthman ibn al-Affan and Ali ibn Abi Talib. The period of these first four caliphs is known as al-khulafā' ar-rāshidūn: the Rashidun or rightly guided Caliphate. Under the Rashidun Caliphs, and, from 661, their Umayyad successors, the Arabs rapidly expanded the territory under Muslim control outside of Arabia. In a matter of decades Muslim armies decisively defeated the Byzantine army and destroyed the Persian Empire, conquering huge swathes of territory from the Iberian peninsula to India. The political focus of the Muslim world then shifted to the newly conquered territories.