Conventions, particularly Republican conventions, usually attract protests and violence. But where Donald Trumpt goes, violence all too often follows. And this convention has become a destination for extremists of all stripes. The killings of police officers in Dallas and Baton Rouge has people worried that even a peaceful protest can be a cover for extremists, particularly because anyone can bring a gun to Cleveland. The Cleveland police, feeling unsupported, might lash out. And, perhaps most important, predicting violence (like this) might become a self-fulfilling prophecy.