For decades, U.S. and international leaders have tried to keep nuclear weapons in the smallest possible number of hands. The more countries that have them, the thinking goes, the greater the chance they could be used in war—or even in a terrorist attack.
To limit nuclear proliferation, countries and international bodies have wielded many policy instruments, from treaties and sanctions to security guarantees and military force. This effort has tallied many successes. But new countries have continued to develop nuclear weapons, and the danger these weapons pose remains.