Thirty-five years ago, governments acknowledged the threat posed by the spread of nuclear weapons and agreed on a Nuclear Nonproliferation Treaty (NPT). Negotiations were spurred by the fear that the number of nuclear powers was set to expand exponentially; rather than a world of five nuclear "haves," there might be dozens. The NPT worked: Today, there are still only five nuclear-weapons states, and there are just three "gray states" that are believed to have weapons outside the NPT framework.

In recent years, the treaty's loopholes have become increasingly apparent. Fortunately, the NPT has a mechanism that allows for periodic review. The most recent such conference just concluded. Unfortunately, it was a failure. Deep divisions among treaty signatories prevented any action to plug those holes. The collapse of the review conference does not herald the end of the NPT, however. It does mean that concerned governments must redouble efforts to find consensus on ways to strengthen the global nonproliferation regime.

It is estimated that there are over 30,000 nuclear weapons scattered throughout the world. Five countries have the overwhelming majority of those weapons: the United States and Russia have most of those, but China, Britain and France are also among the NPT nuclear-weapons states. Three other states -- India, Pakistan and Israel -- are widely assumed to have arsenals of their own. North Korea and Iran are trying to build their own nuclear weapons.