LONDON – We were standing in Hiroshima looking at a stonewall. All there was to see was a shadow of a man. It had been etched into the wall at the moment of his obliteration by the blinding light of the first atomic bomb. Olof Palme, prime minister of Sweden, stared hard at it. An hour later he had to give a speech as head of the Independent Commission on Disarmament of which I was a member. “My fear”, he remarked, “is that mankind itself will end up as nothing more than a shadow on a wall.” Charles de Gaulle once observed, “After a nuclear war the two sides would have neither powers, nor laws, nor cities, nor cultures, nor cradles, nor tombs.” Nikita Khrushchev, who presided over the Soviet Union in the days of the Cuban missile crisis, later wrote, “When I learned all the facts about nuclear power...