The Islamic State’s ideology exerts powerful sway over a certain subset of the population. Life’s hypocrisies and inconsistencies vanish in its face. Musa Cerantonio and the Salafis I met in London are unstumpable: No question I posed left them stuttering. They lectured me garrulously and, if one accepts their premises, convincingly. To call them un-Islamic appears, to me, to invite them into an argument that they would win. If they had been froth-spewing maniacs, I might be able to predict that their movement would burn out as the psychopaths detonated themselves or became drone-splats, one by one. But these men spoke with an academic precision that put me in mind of a good graduate seminar. I even enjoyed their company, and that frightened me as much as anything else.