Modernity requires the willingness to be offended. And as anti-American violence across the Middle East and beyond shows, that willingness is something the Arab world, the heartland of Islam, still lacks. Time and again in recent years, as the outside world has battered the walls of Muslim lands and as Muslims have left their places of birth in search of greater opportunities in the Western world, modernity — with its sometimes distasteful but ultimately benign criticism of Islam — has sparked fatal protests. To understand why violence keeps erupting and to seek to prevent it, we must discern what fuels...