The 1400-year-old schism between Sunnis and Shi'is is currently reflected in the destructive struggle for hegemony between Saudi Arabia and Iran - with no apparent end in sight. But how did this conflict begin, and why is it now the focus of so much attention? In this definitive account, John McHugo charts the history of Islam from the lifetime of the Prophet Muhammad to the present day. He describes the conflicts that raged over the succession to the Prophet, how Sunnism and Shi'ism evolved as different sects during the Abbasid caliphate, and how the rivalry between the empires of the Sunni Ottomans and Shi'i Safavids ensured that the split would continue into the modern age. In recent decades, this centuries-old divide has acquired a new toxicity resulting in violence across the Arab and Muslim world.