T. Dart Walker’s illustration shows Big Jim Parker moving to restrain the assassin, who conceals his pistol in a linen bandage while shooting William McKinley (Image Credit: Library of Congress)

On this day in 1901, President William McKinley stood in the Temple of Music at the Buffalo World’s Fair, gladhanding a series of well-wishers while an organist played a majestic Bach sonata.

To discourage assassination attempts, the Secret Service normally made sure everyone’s hands were empty before they approached the president. But on that hot summer afternoon, the president’s bodyguards let the crowd of overdressed…