Why did President Reagan overthrow SALT II? Three possible reasons.

1. Strategic: so as to save, at the end of this year, a few obsolete Poseidon submarines that would otherwise have to be cut up to keep the United States within the SALT limits. This theory is implausible. The payoff is too small.

2. Psychological: to puncture the mystique of arms control. Here the payoff is big. Hence we have a plausible motive. And a suspect: the powerful faction within the administration led by Richard Perle. It believes that arms control is a paper umbrella under which those who cannot face the exigencies of the nuclear age huddle for false comfort.

The point of renouncing SALT II is thus to break the arms control spell. To say: as of this week and for the first time in 14 years, we are living without arms control. Are you less safe today than you were four weeks or four years ago? The point is to refute the conventional wisdom that arms control is indispensable to our safety and national security.

sk,3 It is a profound objective, and a bad one. There is something to the argument that overselling arms control as an East-West panacea can lead to moral and intellectual demobilization in the West. But that is an argument against overselling, not against arms control.

It is true that arms control can only regulate, it cannot abolish, the nuclear balance of terror. But arms control opponents are simply wrong to believe that we will be better without it. Safer? The ultimate safety in the nuclear age lies in the stability of deterrence. While SALT treaties have not reduced the level of arms, they do provide predictability and limits, no small ingredients for managing deterrence.

sk Stronger? Democracies cannot win arms races. Given the choice, electorates will always choose butter over guns. The reason we are far behind the Soviets in land-based missiles, conventional forces, tanks and almost every category of weapon is not any treaty constraint, but the constraint of the democratic process. Democracies are inherently incapable of sustaining a war footing in peacetime. Arms control is a way to harness that incapacity and make it bilateral.

3. Tactical: The president is trying to portray the SALT II decision as a challenge to the Soviets not to arm but to parley. He is showing the Soviets that he, unlike his predecessors, is politically strong enough and ideologically committed enough to walk away from arms control if necessary. Presumably such a demonstration will move the Soviets to be forthcoming at the bargaining table.

If so, then this is the latest addition to a tall stack of bargaining chips that the president is hoarding. He's got the MX. He's got his Strategic Defense Initiative. Now he has overthrown SALT II. If Reagan's tactic is to concentrate Soviet minds on the wisdom of the kind of deep offensive cuts he says he wants at Geneva, Gorbachev has news for him. The tactic worked.

The Soviets have come back with a very interesting strategic arms proposal. In outline, it is a grand compromise. The United States gets what it wants: deep (one-third) cuts in offensive weapons on both sides. The Soviet Union gets what it wants: severe restrictions on defensive weapons.

The Soviets' offer is a significant move toward the American position. They have given up their usual demand (always dropped when they start to get serious) that the United States count among its strategic weapons the bombers based on aircraft carriers and foreign air bases (such as the F-111s used in the Libyan raid). They have also withdrawn their demand that long-range sea-launched cruise missiles be banned.

Most important is SDI. Rather than try to abolish it outright, they propose a compromise (advanced, incidentally, by The Economist 15 months ago). Both sides would pledge not to withdraw from the ABM treaty -- which bans testing and deployment of defensive weapons -- for 15 years. (The treaty currently permits withdrawal with six months' notice. The Economist suggested a more reasonable three-to-five-year warning period.)

Are the Soviets serious? It is impossible to tell for sure because their offer contains a few fishhooks that they know the United States cannot swallow. By including fishhooks, and deciding later whether to withdraw them, the Soviets are able to postpone the decision of whether they really want a deal or just good press.

But the issue today is not Soviet seriousness. We will find out about that later. (The fishhook test.) The real issue now is Reagan's seriousness. The next few months are his moment of truth on arms control. Over the past five years, he has collected bargaining chips. It is time to use them.

There is much to argue about in the Soviet proposal. But the outlines of a grand compromise are in sight. The question is: Does the president want one? If he does, then we can say that abolishing SALT II was a spur to achieve something better. If he doesn't, then we can say that abolishing SALT II was an American declaration that the age of arms control is over. It is an age on which we will look back with some nostalgia.