For decades, U.S. and international leaders have tried to keep nuclear weapons among as few countries as possible. 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.

Countries and international organizations have tried many kinds of policies to limit nuclear proliferation, from treaties and sanctions to security guarantees and military force. While this effort has led to some successes, the danger remains: countries with nuclear weapons capabilities have not yet disarmed and those previously without nuclear weapons are developing them. Over time, countries have narrowed down a few strategies to enforce nonproliferation, though the debate over which ones are best continues.

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