I’ve spent the last few years in Barcelona studying radicalization. As the day of the terrorist attacks in Spain unfolded, I thought, what comes next? The blaming of the Muslim community, the demonizing of the town the attackers came from, and vows from politicians to throw more money at the problem. But my time in Barcelona taught me one thing: radicalization is a local phenomenon. Equipping local officials to solve local problems—and avoiding the distraction of easy, unhelpful generalizations about immigrant or local communities—is the best way to thwart the jihadists’ international aims.