I would change the Treaty of Versailles that ended World War I. The American President Wilson tried to persuade the allies to be forgiving, but his European counterparts instead stuck it to the defeated, especially Germany, demanding huge reparations that impoverished them and created the conditions for fascism to triumph. The vengeful allies of World War I created Hitler, and if I could, I would change their minds so that Hitler and his Nazis never had the chance to come to power; they would have remained a far-right fringe group in a rebuilt and prosperous Germany.
