In this approaching spring of ever changing moods in Washington -- a new face in Moscow, the resumption of arms control talks, the debate over "Star Wars" and deficit phobia -- our fundamental approach to the Soviet Union badly needs a measure of perspective.

Neither a change of leadership in the Kremlin nor a resumed arms control negotiation is a reliable guide to the future of U.S.-Soviet relations. It would be wishful thinking to believe that these recent events can be isolated from the larger success or failure of our general foreign policy. Whether the connection is termed "linkage" or merely common sense matters little. Nothing has occurred to dispel the reality of continued intractable differences between the United States and the Soviet Union, differences arising from Moscow's continuing proclivity to support wars of national liberation, bloodshed and terrorism to achieve its global aims.

What has occurred, however, is that the correlation of forces, as Moscow likes to put it, has been gradually shifting against the Soviet Union. Unlike the situation 10 years ago or even five years ago, the Soviet Union now finds its ability to expand its world influence increasingly constrained. Its current dilemma derives from a complex combination of its own failures and the revival of American will and strength already evident in the last year of the Carter administration and given greater emphasis under President Reagan. This turn of events, not the president's rhetorical excesses, has been the major contributor to Moscow's sourness in the Soviet- American dialogue.

Now is certainly not the time to mistake momentary optimism about Soviet behavior for lasting success -- especially not at the beginning of what may be a very vigorous and long-lived Soviet leadership. Moscow's forces are too strong and its hostility to freedom too enduring for us to believe that a younger man in the Kremlin and the improved atmospherics mean a safer world.

Still less can we imagine, as some rhetoric suggests, that the concept of deterrence has somehow become immoral or ineffective and therefore should be replaced. To the contrary: A major contribution to the shift in the correlation of forces is what we have done to strengthen deterrence. We have added to the constraints on Soviet use of force and enlarged the margin of safety for free peoples -- and for those struggling to be free. Our purpose, both in our policy and through the arms control negotiations, should be to strengthen deterrence further.

To do so, we must understand what we want deterrence to achieve. It is to prevent war by persuading those who make the calculus of risk in Moscow that the use of aggressive force, at whatever level, would cost far more than what it might achieve. For the United States as a defensive power, deterrence requires that we be able -- both in fact and in perception -- to counter Soviet threats at every level of conflict. There can be no substitute for such a capability. At a minimum, Moscow must be uncertain about a possible U.S. response, whether conventional, theater nuclear or strategic nuclear in character.

Today, that uncertainty depends in large part upon offensive nuclear forces. Unfortunately, America's current arsenal remains deficient in a major category: land-based ballistic missiles capable of promptly destroying hardened targets. After a decade of debilitating debate, the bipartisan Scowcroft commission suggested in 1984 a solution that includes rapid deployment of 100 MX missiles and the longer-term development of a new, smaller mobile missile, Midgetman. The commission's conclusions about the MX did not derive from any particular admiration for that weapon. The weapon is simply the best we can do to strengthen our deterrent capability as soon as possible, to augment the shift in the correlation of military forces that plays its part, along with our diplomacy, in constraining Soviet ambitions. This does not mean that the concept of deterrence must depend for all eternity on offensive missiles. But both critics and supporters of the president's Strategic Defense Initiative -- and I am a supporter -- should be guided by the ultimate purpose of our weapons: to prevent war. Far better to prevent war itself than to debate theories of how we are going to win it.

That is why we must not reach hasty conclusions about the future of strategic defenses at the expense of current efforts to strengthen deterrence. We should not lull ourselves into the belief that a few years hence some technological miracle will somehow solve our problems. Still less should we let the Soviets focus our attention on an unrealized research program rather than the pres deployed Soviet ICBMs capable of hitting hard targets.

Nor should we succumb to a "bargaining chip" thesis, which takes our present remedies to strategic imbalances and holds them hostage to the outcome of a negotiation. Such a negotiation can only succeed in serving our national security if it ratifies a stable deterrence or if it contributes to it by helping to stabilize the calculus of risk in Moscow against any use of force.

Finally, a word on psychology. Nations do not live on arms alone, and the peace is not preserved simply by the accumulation of weapons. Deterrence depends upon our perceived will to act as well as on our physical capabilities. The successful deployment of NATO's Pershings and cruise missiles and the sacrifices made by the American people for defense, despite economic hardship, have given the lie to those who doubted the determination of the democracies to defend themselves. We must continue on this path in order to dash the hopes of those who believe that our recent resolves are only a temporary detour on the road of business as usual.

Even as we make the necessary improvements to our defenses, no one can doubt the deep yearning of our people to be free of the nuclear danger. The "peace movement" is one manifestation of this yearning. President Reagan's defense initiative, whose objective is to render nuclear weapons "impotent and obsolete," has also appealed to a new generation whose horizons have been clouded by the fear of nuclear war.

It does no disservice to the vision of a non- nuclear war, however, to insist upon historical perspective. When British Prime Minister Thatcher addressed our Congress recently, she quoted Winston Churchill's advice of more than three decades ago: "Be careful above all things not to let go of the atomic weapon until you are sure and more than sure that other means of preserving peace are in your hands." A non-nuclear world, as our fathers and our grandfathers learned to their sorrow, is not necessarily a peaceful one. Improved deterrence is our only choice to prevent war, including a nuclear war. A calculus of risk that precludes aggressive use of force continues to be the only formula to keep the peace, even in a non-nuclear world.