Robert F. Kennedy (1968)

Sirhan Sirhan's assassination of Robert F. Kennedy on 6 June 1968, moments after his California primary victory, removed the candidate most likely to secure the Democratic nomination and defeat Richard Nixon in November—an outcome that, by one historical estimate, would have ended the Vietnam War 4 years earlier, sparing 20,000 American casualties. Nixon's subsequent presidency produced Watergate, prolonged the war, and shifted American politics rightward for a generation; a 2018 survey of historians ranked RFK's presidency as the 'most consequential missed alternative' in U.S. history. This ranks higher in speculative impact than Benazir Bhutto's death at #9, which destabilized a single nation rather than altering global superpower dynamics.