Einstein urged the building of the atomic bomb—and later became a proponent of nuclear disarmament.

In the late-1930s, Einstein learned that new research had put German scientists on a path toward creating the atom bomb. The prospect of a doomsday weapon in the hands of the Nazis convinced him to set aside his pacifist principles and team up with Hungarian physicist Leo Szilard, who helped him write a letter urging President Franklin D. Roosevelt to conduct atomic research. Though Einstein never participated directly in the Manhattan Project, he later expressed deep regrets about his minor role in brining about the Hiroshima and Nagasaki bombings. “Had I known that the Germans would not succeed in producing an atomic bomb, I never would have lifted a finger,” he told Newsweek. He went on to become an impassioned advocate of nuclear disarmament, controls on weapons testing and unified world government. Shortly before his death in 1955, he joined with philosopher Bertrand Russell in signing the “Russell-Einstein Manifesto,” a public letter that stressed the risks of nuclear war and implored governments to “find peaceful means for the settlement of all disputes between them.”