Until the nineteenth century, religion was usually the only acceptable justification of terror. It is not hard to understand why: religion gives its true believers an account of the good life and a way of recognizing evil; if you believe that evil in the form of wrong beliefs and mistaken customs weakens or corrupts a life ordained by God, you are under a profound obligation to combat that evil. If you enjoy the companionship of like-minded believers, combating that evil can require that you commit violent, even suicidal, acts. The Thuggees of India during their several centuries of existence may...