evidence: Political violence is a core strand of the national story. It is as American as apple pie. What we witnessed on Saturday evening, when the shots rang out in Pennsylvania, was not some freak event. Deadly violence has been a recurring feature of presidential politics. [...] President Lyndon Johnson wanted to explore why America was plagued by so much political violence. The panel of scholars he appointed to undertake this quest for understanding concluded racism was a primary cause. So, too, the "frontier experience" of suppressing Native Americans and Mexicans, which created a tradition of vigilante justice. The manner of America’s founding, through glorious revolution, also made it prone to violence. But memory loss added to the problem. Like all nations, their report concluded, the US suffered from "a kind of historical amnesia or selective recollection that masks unpleasant traumas of the past, but Americans have probably magnified this process of selective recollection, owing to a historic vision of ourselves as a latter-day chosen people, a new Jerusalem".