" The religious roots of this struggle go back to the seventh century and the battle that ensued after the death of the Prophet Mohammed. Who would be his successor—his father-in-law, Abu Bakr, or his cousin and son-in-law, Ali? The Sunni are followers of Abu Bakr, who became the first caliph. But his legitimacy was—and is—contested by the Shia, “the party of Ali.” Each group regards the other as heretics. "

― Daniel Yergin , The New Map: Energy, Climate, and the Clash of Nations