The images are remarkably familiar. The convertible limousine winds its way through Dallas crowds; John F. Kennedy, America's youngest president, smiles and waves in the backseat; gunfire, three jerks, the limousine slows and then accelerates; Jackie Kennedy shrieks and covers her husband; an emotional Walter Cronkite tells the nation that its president has died, removes his glasses, and wipes his eyes.

On November 22, 1963, President John F. Kennedy was shot as he paraded through Dallas, Texas. That same afternoon, Dallas police arrested their suspect, Lee Harvey Oswald, an itinerant, self-described "Marxist-Leninist" who had lived in the Soviet Union. Within days, Oswald was also dead, shot by club-owner Jack Ruby on national television in the basement of a Dallas police station.

Kennedy's election to office marked, for some commentators, a new age in American political culture. "History with a capital H had come down to earth, either interfering...