Explaining ISIS

Professor Bernard Haykel’s succinct explanation of ISIS (cover story, June 3) is helpful, yet I’ve heard suggestions that its leaders go beyond a “strict literalist interpretation” of the Quran’s text and the Prophet’s sayings to justify their brutality by citing an even more “original” (or in Arabic, Salafist) authority, including the so-called “river of blood” battle of A.D. 633, when a revered Caliphate commander promised Allah that, if he gained victory over Persian and Arab Christian warriors, he would create from his captives a river of blood in the Euphrates, as indeed legend claims he did. Shouldn’t this be included in the explanation?