Breaks the nuclear firewall.

Japan has tons of bomb-grade plutonium, left over from its atomic-energy program, and could build a small nuclear arsenal within a year. South Korea could do the same in perhaps two years, and on January 11, its president raised the possibility that his country might need to “possess its own nukes.” Japan and South Korea now face nuclear threats from North Korea and China. More than 70 percent of South Koreans think their country should obtain nuclear weapons, and Japan has decided to double the size of its military budget. Taiwan could have its own nuclear weapons within a few years of deciding to build them. Saudi Arabia could also obtain them quickly. At a conference in Abu Dhabi this December, the Saudi foreign minister made clear that “if Iran gets an operational nuclear weapon, all bets are off.” And if Saudi Arabia gets nuclear weapons, Turkey, Egypt, and Algeria might build them soon too.

On a number of occasions during the Cold War, the United States and the Soviet Union came perilously close to a nuclear conflict that neither side wanted—a conflict that could have killed hundreds of millions of people. It is remarkable that no city has been destroyed by an atomic blast since Nagasaki in 1945. The spread of nuclear weapons to more countries, amid today’s rising nationalism and bitter ethnic hatreds, would no doubt increase the likelihood of mushroom clouds rising over the rubble of cities.