After the death of Abu Bakr, disputes over succession led to the establishment of the Umayyad dynasty in 661. The Umayyads expanded the Islamic empire significantly but faced revolts due to discrimination, splitting Islam into the Sunni and Shia sects. In 750, the Abbasid dynasty overthrew the Umayyads and moved the capital to Baghdad, ushering in a period of prosperity. However, regional rulers gained independence and the Fatimid dynasty was later overthrown by the Seljuk Turks, leading the Byzantine emperor to request Christian assistance and launching the Crusades between Christians and Muslims.