An arms control primer for those trying to make sense of it all. A look at the key issues (with a minimum of acronyms)

The one sure thing about arms control is that it is confusing. But with the chilly air of the Iceland meeting reaching the ongoing talks in Geneva, it's especially important to try to sort out the issues. Here are a few basics that might help in the hectic next few months, as the East-West debate over arms issues heats up.

Q. Why do nations have nuclear weapons?

A. Because they are the most destructive weapons known to man, and if the other guy has them you want to have them too.

They have only been used once -- to hasten the end of World War II. That singular use was by a nation holding a nuclear monopoly, the United States, against a nonnuclear nation, Japan. They are, we hope, unlikely to be used again (some calculate the odds at about 2 percent), now that both superpowers have them. The main aim now is not to use them but to prevent the other side from using them. That is, the consequences of a retaliatory strike threaten to be so devastating that neither side wants to attack and risk mutual suicide.

Q. Then do nuclear weapons give nations security?

A. Yes and no. Minimum security is possible -- that is, ``deterring'' the other side from using its nuclear weapons out of fear of your retaliation. Maximum security has never been possible -- that is, guaranteeing that one's own side would survive a nuclear exchange with a society that could pick up the pieces and go on.

Q. Would ``star wars'' (the Strategic Defense Initiative, or SDI) change this?

A. President Reagan's original hope was that SDI would make it possible to get rid of this uncomfortable ``deterrence'' and get back to good old-fashioned ``defense'' -- shooting down incoming missiles rather than just deterring the Soviets from launching them in the first place.

Those working on the SDI program now generally talk of a more limited aim for SDI, however. Intercepting all incoming missiles to protect populations looks pretty impossible, technologically and financially. Instead, the idea now is more that SDI would be used as a highly selective umbrella to protect missiles, command posts, and other priority targets. Thus it would not supplant 40 years of nuclear ``deterrence'' with a new regime of ``defense,'' but would strengthen deterrence by assuring survival of ``second-strike'' retaliatory weapons.

Q. Wait a minute. Aren't retaliatory weapons already designed to survive a surprise attack?

A. Yes and no. Submarines and slow-flying airplanes still are ``survivable,'' more or less. But fixed land-based missiles -- like the American Minuteman and the Soviet SS-18 -- have been becoming increasingly vulnerable, since the invention of MIRVs (multiple, independently targetable warheads on single missiles) in the 1970s.

In the days of single-warhead missiles, an aggressor would lose more of his own missiles than the enemy missiles he would knock out in any attack. An attack, therefore, made no sense. After the US introduced MIRVs, however, an aggressor could theoretically knock out the bulk of the adversary's ICBMs (land-based intercontinental ballistic missiles) with only a modest number of missiles.

Of course such action would be a wild gamble, given the remaining enemy warheads on subs and planes. But in a crisis, a superpower might figure it would lose less by taking this dangerous gamble. For if the other side were to shoot first, it could wipe out the superpower's most accurate and powerful weapon, the ICBM.

Q. Where does arms control fit into this?

A. Arms control could put a ceiling on the number of warheads relative to missile launchers and other ``hard targets'' and bring it back down toward the pre-MIRV ratio. Then we would be back to where a first strike wouldn't make any sense. Even in a crisis, each superpower would wait to see what the other was doing.

Q. So arms control would turn the clock back to the time of Nixon's first Strategic Arms Limitation Treaty (SALT I)?

A. Precisely. The deep ``50 percent cuts'' that are now being talked about (which are actually more like 40 percent, but never mind) would only bring things back down to the numbers of the early 1970s. But they could banish the nightmare threat of a surprise attack by restoring a situation in which an attacker would lose more than he would gain by a ``first strike.''

Q. And where does SDI fit into that?

A. You can't just glue the MIRVs back into single warheads again. You are still going to have more warheads than targets, whatever you do. Therefore, some strategists think a fairly simple defense might be needed to get the ratio back down to manageable proportions. If you could intercept, say, half the warheads aimed at a target, this would double the number of warheads an attacker would need to use to wipe out that target. It might not be worth it to launch an attack.

Q. If ``50'' percent cuts in offensive strategic weapons are good, then would 80 or 100 percent be better?

A. No, they would be worse, for two reasons.

First, you would get into a situation in which cheating would be much more rewarding. If both sides have 6000 warheads, 50 hidden warheads won't make a difference. But if both sides have zero, the side that secretly produces 50 is king.

Second, if launcher numbers go much below the 1600 now being discussed, the ratio of warheads to hard targets would increase and thus potentially increase the incentive for first strike. It's rather complicated, but all the number crunchers say that ``50'' percent cuts are about as low as you can go before you once again begin to produce instability and itchy trigger fingers.

Q. That flies in the face of common sense.

A. Right. A lot of things in the nuclear age do.

Q. But what if we don't make it in arms control? Is the alternative an ever more destabilizing all-out arms race in both offensive and defensive weapons?

A. Opinions differ. Arms controllers tend to believe this would be the case, and that it would be a bad thing. Hawks also tend to believe this is likely, but that it would be a good thing, since the Americans would win the technology race.

Some other strategists argue that nuclear deterrence has kept superpower peace for 40 years now and is a lot sturdier than we give it credit for.

They believe that these weapons are so awful that their mere possession by both superpowers deters their use, and all the arcane counting of warheads and targets doesn't make much difference.