Speaking at the Ninestiles School in Birmingham, Cameron laid out his theory of "Islamic extremism" — what it is, how it works in general, how it works in the context of the Islamic State, why it attracts people around the world to its cause, and how to fight it in the UK.

Cameron's main argument is this: "The root cause of the threat we face is the extremist ideology itself." Want to understand the Islamic State? Look at its ideas, or "extreme doctrine," as Cameron puts it. 

That would be a totally reasonable thing to argue, except in this case, Cameron's singular focus on ideology became its own kind of extreme doctrine — one that largely dismissed social, economic, and historical explanations for the rise and appeal of the Islamic State.