Until modern times, there existed no form of legitimacy in the Middle East outside of Islam. Rulers ruled in the name of God; assassins struck them down in the name of God. The assassinations of the early caliphs and the struggle between the Sunni rulers and the Assassins in the Middle Ages took precisely this form: each side claimed to act in accord with divine will, revealed in divine texts. Religion played a crucial role in the rationale of assassination, but it also played a crucial role in the rationale of government, law, and warfare—indeed, of everything. This invocation of...