The American poet Robert Frost famously mused on whether the world will end in fire or in ice. Nuclear weapons can deliver both. The fire is obvious: modern hydrogen bombs duplicate on the surface of the earth the enormous thermonuclear energies of the Sun, with catastrophic consequences. But it might be a nuclear cold that kills the planet. A nuclear war with as few as 100 hundred weapons exploded in urban cores could blanket the Earth in smoke, ushering in a years-long nuclear winter, with global droughts and massive crop failures. The nuclear age is now entering its seventh decade. For most of these years, citizens and officials lived with the constant fear that long-range bombers and ballistic missiles would bring instant, total destruction to the United States, the Soviet Union, many other nations, and, perhaps, the entire planet. Fifty years ago, Nevil Shute’s best-selling novel, On the Beach, portrayed the terror of survivors as they awaited the radioactive clouds drifting to Australia from a northern hemisphere nuclear war. There were then some 7000 nuclear weapons in the world, with the United States outnumbering the Soviet Union 10 to 1. By the 1980s, the nuclear danger had grown to grotesque proportions. When Jonathan Schell’s chilling book, The Fate of the Earth, was published in 1982, there were then almost 60,000 nuclear weapons stockpiled with a destructive force equal to roughly 20,000 megatons (20 billion tons) of TNT, or over 1 million times the power of the