Rhodes, who won the Pulitzer Prize for his book The Making of the Atomic Bomb, writes that Richard Oppenheimer, leader of the U.S. effort to develop the nation's first nuclear weapons at Los Alamos, New Mexico, saw the spread of technology as inevitable and the threat of nuclear weapons as inescapable. But Oppenheimer and colleague Niels Bohr hoped the world's nations would cooperate to limit the weapons--a hope that materialized in the nuclear Non-Proliferation Treaty (NPT) that went into effect in 1970. The treaty binds its 188 participating nations to work toward nuclear disarmament, refrain from transferring nuclear weapons to non-nuclear-weapon states and to share nuclear technology for civilian uses. Other agreements have further limited the spread of nuclear weapons.

The collapse of the Soviet Union, which held the world's largest nuclear arsenal, presented a new nuclear threat, Rhodes points out. Former Soviet states have relinquished the nuclear weapons they acquired with the USSR's dissolution, but some nuclear material may still be unsecured. Meanwhile, terrorist groups aspire to the ultimate weapon, and NPT nations are considering new agreements to prevent terrorists from getting bomb-making materials.

"Niels Bohr and Robert Oppenheimer would recognize" today's nuclear dilemma, Rhodes says, and "their advice . . . still holds: Only cooperation among nations can secure the deadly metals from which nuclear weapons are made. Only negotiated reductiosn in arsenals and limitations on weapons development can diminish the long-term risk to us all."

A sidebar to this article, "Ground zero: Scenarios for a nuclear attack," by Lynn Eden and Alexander Montgomery, describes what would happen if Washington, DC, were attacked by nuclear weapons of three different types. Both articles are available online to National Geographic subscribers.