On my recent trip to Russia, I spent an hour with Mikhail Gorbachev. I told him that in the West we are grateful that he and President Ronald Reagan defied Cold War orthodoxy to significantly reduce our countries’ nuclear arms. And I asked him whether there was a moment in his life when he’d realized that he might shape history. He paused a moment and then recounted how as a young man, he had watched a film on the devastation that would occur with nuclear war. He and the other young officials in the room looked at each other in shock as the film concluded. Gorbachev recalled the scene: “Even...