Immediately after the assassination and ever since, the right has tried to deflect any connection between its fevered Kennedy hatred and Oswald’s addled psyche with the fact that the assassin had briefly defected to the Soviet Union. America’s violent culture wars had started before JFK was shot. They were all on display in Oswald’s Dallas. At least in 1963, polling showed that only 5 percent of the country—a fringe—subscribed to the radical anti-government views championed by the John Birch Society and other militants of the right. These days, that fringe, whether in the form of birthers or the tea party...