When israel was working on a secret nuclear program in the 1960s, satirist Tom Lehrer captured the rationale succinctly: "'The Lord's our shepherd,' says the psalm, but just in case, we better get a bomb." Israel was following the U.S., Britain, the Soviet Union, France and China. India joined the club too, followed eventually by Pakistan. And today North Korea, Iran, Iraq and Libya are knocking at the door.

Some see these moves as steps toward a world full of nukes. But in retrospect what is striking is less the number of countries that have gone nuclear than the number that have not. And several, including South Africa, Brazil and Argentina, have even reversed course and abandoned once active weapons programs.

Will such restraint continue for long in the anarchic, postÐcold war era? American officials hope so, but they aren't betting on it. Now as before, when it comes to proliferation, the U.S. tries to have it both ways. It supports a wide range of arms-control efforts designed to stop the spread of nuclear weapons abroad while hanging on tightly to a nuclear capability of its own and sometimes even brandishing it to make a point. This attracts criticism from others, but hypocrisy is par for the course in international relations. And besides, American officials are correct to argue that in this case consistency—"Hey, everyone should have as many bombs as we do!"—would not be a virtue.

Looking ahead, old-style proliferation—the acquisition of nuclear weapons by non-nuclear states—will hardly be the worst problem. Most countries have little interest in getting the bomb, either because they are not gravely threatened (such as Costa Rica) or because their safety is guaranteed by another nuclear power such as the U.S. (like Germany and Japan). The few countries that are interested, meanwhile, can be divided into what have been called the orphans and the rogues.

Orphans such as Israel, India and Pakistan live in dangerous neighborhoods and have legitimate security concerns. They will probably behave just as the original nuclear powers did, which is to say they will use the weapons primarily for deterrence. The ultimate effect of their joining the club should be to extend the cold war's great power stability—and harrowing crises—to a few regional hot spots. The chief problem with the orphans is getting them to understand the importance of proper safety measures, secure command and control procedures, and other cold war lessons. Rogues such as Iraq, North Korea and Libya are much more dangerous because they might be reckless or desperate enough to threaten or use their capabilities for offensive and not merely defensive purposes. Keeping weapons of mass destruction out of their hands is a critical challenge, which will have to be met by constant bullying—and occasional bribing, along with better control over the materials and expertise from the former Soviet nuclear program. Just as critical will be maintaining a strong and credible nuclear and conventional deterrent so that even if rogues should manage to get terrible weapons, they will think long and hard before using them.

The really difficult problem will be a new kind of proliferation involving the acquisition of chemical, biological and cyberweapons by subnational actors such as terrorist groups, cults or angry individuals. These weapons are easy to make, hard to track and hard to defend against. This means that even if the U.S. does spend tens of billions of dollars on a system to shoot down North Korean missiles, we will still have to deal with the equally pressing problem of stopping doomsday cults or future Unabombers armed with deadly viruses. Just as internecine conflicts have risen while interstate wars have declined, so subnational terror may increase as traditional nuclear threats wane. Monitoring and deterring states are hardly easy, but they pale in difficulty compared with the task of monitoring and deterring obscure and fleeting groups and individuals. Not everyone in our future will have a bomb, of course. If countries really want to, though, a lot of them will still be able to lay their hands on the stuff of nightmares.