The world sleeps easier since the end of the superpower competition and its accompanying threat of nuclear annihilation, but fears that a rogue state or terrorist group might acquire nuclear weapons have grown. That concern has been magnified by the increasingly visible failings of the global nonproliferation order.

Four decades ago, there were fears that the world would have two dozen nuclear weapons states. That dark vision did not come to pass. Today, there are only five "recognized" nuclear-weapons states -- Britain, China, France, Russia, and the United States -- and three "gray" states -- India, Israel and Pakistan -- who have such weapons but are not recognized by the international community as "nuclear-weapons states." Several countries that could have developed nuclear weapons did not. One country, South Africa, developed them and then destroyed them. Ukraine and Kazakstan inherited nuclear weapons with the breakup of the Soviet Union, but gave them up rather than keep them.

The primary instrument that has checked the development of nuclear weapons is the Nuclear Nonproliferation Treaty (NPT). The treaty was founded on two bargains. In one, the nuclear-weapons states guarantee nonnuclear states the right to the peaceful use of atomic energy in exchange for those same nonnuclear states giving up their claim to possess nuclear weapons. By and large, that bargain has been honored, although it has become increasingly apparent that a government can cheat. A determined proliferator can claim to pursue the peaceful use of nuclear energy, acquire critical technologies and then "break out" and develop nuclear weapons. Iran is accused of doing just that, although the government in Tehran denies the charge.