"The sects and factions that formed during the first and second fitnas gave birth to what we now know as the Sunni-Shia divide. Shia Muslims refused to accept the legitimacy of the Umayyad caliphate, or indeed the legitimacy of Abu Bakr, Umar, and Uthman’s regimes. Instead, they insisted that Ali was Muhammad’s rightful successor: the first imam. This in turn implied an alternative succession, through Hasan and Husayn, then a bloodline of further imams descended from Muhammad. Now this was not solely a dynastic dispute. The Shia framework of Islamic history proposed a significantly different model of organizing the umma, and a different set of leadership values. The Sunni-Shia divide came to be tremendously important during the later Middle Ages, particularly (as we shall see) during the crusading era. But it has lasted far longer than that. During the twentieth century, a revived, poisonous sectarianism established in part along Sunni-Shia lines began to inform world geopolitics—playing a role in the interconnected Iran-Iraq War, U.S.-led Gulf wars, and long-running “Islamic cold war,” which has pitted Saudi Arabia and Iran against one another for regional hegemony in the Middle East since 1979; as well as other, painful and deadly conflicts that have been fought in Pakistan, Iraq, and Syria. That all this can still be traced back to the machinations of powerful men in the seventh century A.D. may seem astonishing—but as so often proves the case, the Middle Ages remain with us today."