...Our ignorance of the Islamic State is in some ways understandable: It is a hermit kingdom; few have gone there and returned. Baghdadi has spoken on camera only once. But his address, and the Islamic State’s countless other propaganda videos and encyclicals, are online, and the caliphate’s supporters have toiled mightily to make their project knowable. We can gather that their state rejects peace as a matter of principle; that it hungers for genocide; that its religious views make it constitutionally incapable of certain types of change, even if that change might ensure its survival; and that it considers itself...