This week has become a grotesque object lesson in gun culture, one that points to a conclusion that we could have and should have drawn long ago—that the surfeit of weapons at our disposal and the corresponding fears that they induce create new hazards. There is no telling how any of these specific horrors will be resolved. But here is what we do know: we live in an age of open-source terrorism. Our inability to respond to mass shootings has meant that, eventually, even law enforcement would fall victim to one. The context of the conversation about police accountability has been irrevocably changed. Black lives matter, but reports that those words were uttered by a gunman in Dallas mean that any movement under that banner may well have met its end. And realism, in the face of tragedy, tells us that there is more ugliness in the offing.