IN 1963 President John F. Kennedy famously speculated that by 1970 ten countries would be able to deploy nuclear weapons and by 1975, fifteen to twenty countries would have followed suit.¹ In fact, the number of nuclear-weapons states peaked at ten when some of the successor states to the Soviet Union were born nuclear, then the number dropped to seven, and now it stands at nine with Pakistan and North Korea having joined what remains the world’s most exclusive club.

Why have nuclear weapons spread so slowly? The answer is found not in the Nuclear Non-Proliferation Treaty but in the fact that most countries feel sufficiently secure without adding nuclear weapons to their conventional arsenals. If a country believes that its security depends on nuclear weapons, to prevent it from acquiring them becomes almost impossible. President George W. Bush among others announced that North Korea becoming a nuclear-weapons state would be “unacceptable.” Yet when North Korea developed a nuclear military capability, we quietly acquiesced. The alternative to acceptance would have been to attack North Korea’s nuclear facilities, and that would surely have been unacceptable.

Sagan emphasizes that verification and enforcement of an agreement to create a nuclear-free world would be required. If for a moment we imagine that Sagan’s hoped-for world without nuclear weapons could be realized, what would anyone do if a major state revealed that it had secretly rebuilt a considerable nuclear arsenal? Would someone then attack the reborn nuclear state using the only weapons it would have, that is, conventional ones? I think not.

Sagan, having rejected “legal niceties,” would instead rely on international pieties. If other countries believe that America is on the road to nuclear disarmament, they will presumably jump on the bandwagon. The problem is that no road leads from a world with a small number of nuclear-weapons states to a world with none. Sagan believes that if the world does not become nuclear weapons free it will soon be a world of many nuclear states. One wonders why. Many more states can make nuclear weapons than do. Why should one now expect a large number of new nuclear states to join the hitherto-exclusive club? The old answer was that states seek the prestige that members of the club enjoy. Little prestige, however, attaches to new nuclear states when countries like Pakistan and North Korea already have them. Strong states—the old Soviet Union and China—are no longer seen as threats. Sagan would have us transfer our worries to puny new nuclear states and terrorists. We used to worry about the strong; now we should worry about the weak.

Sagan emphasizes the perils that attend a world with many more nuclear states, thus increasing the chances that terrorists would be able to steal or buy nuclear devices. To find good words to say about terrorists is difficult. Terrorists are a big annoyance and may occasionally do a fair amount of damage. We all know about the attacks on the Twin Towers and the Pentagon in which upwards of three thousand people perished. One thought, however, gives comfort: terrorists are incapable of rending the fabric of society and of occupying and administering territory. We should all heave a sigh of relief that strong adversaries have been replaced by weak ones.

¹ Arthur M. Schlesinger Jr., A Thousand Days: John F. Kennedy in the White House (Boston, MA: Houghton Mifflin, 1965).

Scott D. Sagan is the Caroline S. G. Professor of Political Science at Stanford University and co-director of Stanford’s Center for International Security and Cooperation.

Kenneth N. Waltz is the Emeritus Ford Professor of Political Science at the University of California at Berkeley and senior research associate at Columbia University’s Saltzman Institute of War and Peace Studies. In 1999 he won the James Madison Lifetime Achievement Award from the American Political Science Association.