We like to think Islamism represents only a fringe of the world’s 1.4 billion Muslims. But that’s because we confound Islamists and terrorists. The terrorists — those who commit and materially support violent attacks — are a fringe (bigger than we’d like to think, but still a tiny minority). By contrast, Islamists may be a majority, and, if they’re not, they constitute a very substantial minority. Islamism is not terrorism. To be sure, Islamism includes terrorism in its arsenal. Still, there is major disagreement among Islamists about when violence should be used and how effective it is. In any event,...