IN THE GREAT debate over nuclear weapons and how to control them, one key fact gets too little attention: The Soviet Union depends on its nuclear arsenal not only to protect itself and to threaten others, but for its very status as a great power. Without nuclear weapons, the Soviet Union would not be a superpower.

Perhaps we shy away from this fact because its implications are so depressing. Obviously, if the Soviet leaders believe that they depend on nuclear weapons for their international status, there are unlikely to consider giving them up. This state of affairs cannot be wished way.

More depressing still, the nuclear revolution that has transformed international politics gives the Soviet Union considerable advantages over the West in the competition for global influence. The very "unusability" of nuclear weapons makes them more useful to the Soviets than they can ever be to us.

Nuclear weapons are much more than weapons. They are the overriding political and psychological factor in Soviet-American relations, and in today's global politics generally. The fact that nuclear weapons exist has created an international situation without precedent in history.

The existence of nuclear weapons has also given the United States and the Soviet Union a set of compelling common interests, beginning, of course, with their mutual need to avoid a nuclear war. But common interests are far from either supoerpower's only interests.

If avoiding a nuclear war and the confrontations that could lead to one was both superpowers' fundamental goal, their chances for success would be very high. Even if the nuclear arms race continues indefinitely, one can argue -- however ironically -- that both sides believe they are accumulating new weapons to avoid the sort of strategic imbalance that could tempt one superpower to attack the other.

But the prevention of a nuclear holocaust and the survival of humanity is not the only fundamental goal of either America or of Russia. Both have other international ambitions, and the Soviet Union has an unsatisfied hunger for global influence that pushes it toward a highly activist foreign policy.

It's no use shying away from this uncomfortable fact. Soviet nationalism and pride, history and ideology, its military might combined with the docility of its population, its late arrival on the superpower stage all encourage this activism. The Soviet Union is still in the ascending stage of its international ambitions.

In this stage the Soviet leadership is hoping it will be able to turn Western Europe away from its alliance with the United States. The Soviets are committed to support radical changes in the Third World, even if this involves direct and indirect Soviet military engagement. While the overriding Soviet goal is to protect its own Russian-Soviet system and its "internal" and "external" multinational empire, its key tactical aims as a latecomer to the "game" of global politics are offensive.

For the United States, this is clearly the descending stage of political, economic and military ambitions. America now views the international system with the attitudes of a status-quo power. Its long-term policy direction is best expressed in "containment" -- a concept with basically defensive connotations.

American key goals are the preservation of the independent industrial democracies, which requires a balance of at least strategic parity with the Soviet Union; the defense of vital economic and strategic American and allied interests in such areas as the Persian Gulf, which requires the preservation or creation of a conventional Soviet-American balance in some regions of the world; and finally, the preservation -- at best -- of the existing configuration of forces in the Third World. Where change cannot be avoided, the U.S. wants gradual evolution, not revolutionary upheavals.

This is a dangerous situation. Although each superpower wants to achieve its goals without using nuclear weapons -- that is, in a rational way -- this may not prove to be so easy. The asymmetry of goals which each superpower pursues and the fact that one is still ascendant while the other is retrenching increases the perils on an unintended escalation of their conflict to the threshhold of nuclear war.

To put it simply, the nuclear revolution in weaponry and warfare and its military, political and psychological consequences give the Soviet Union a better opportunity than the United States to achieve its international goals without a nuclear war. This is true for four reasons:

First, nuclear weapons act as a great equalizer of the actual, conventional military potential of the Western and Soviet alliances, and therefore are a guarantor of the security of the Soviet Union. Far from being an equal, the Soviet Union and its bloc are far behind the Western alliance in conventional military potential.

At the beginning of the 1980s the combined gross national product (GNP) of the industrial democracies was more than three times larger than the combined GNP of the Warsaw Pact countries. (This is about the same relationship that existed between the West European and Soviet economic military potential on the eve of World War II, and is only marginally different than the comparative potential of NATO and the Warsaw Pact in the 1950s). Technologically, recent studies have shown that the lopsided superiority of the West is at least as great today as it was in 1953, the year when Stalin died

But nuclear weapons -- strategic, theater, and tactical -- act as a great equalizer of the military potential and actual strength of the competing blocs, who without them are not equal at all. Once strategic parity has been achieved, it provides a degree of security to the Soviet Union that it never enjoyed in the past.

And of course, nuclear weapons enhance the psychological-political effectiveness of any threat from Soviet conventional forces on Soviet borders or in remote corners of the globe.

Second, the nuclear revolution led to the the once-justified but now pernicious concept of an American nuclear umbrella over Western Europe, and the "biger bang for the buck" military policy of the United States. The idea of an American umbrella led to much lower levels of conventional military spending by the industrial democracies than at any time in their 20th-century history. The idea of "more bang," and the attendant American preoccupation with the nuclear balance, led to a deterioration of conventional U.S. forces and to the dissipation of the American military-industrial base. And of course, these changes occured at a time when Soviet conventional forces were growing steadily. The resulting superiority of mobilized conventional strength gave the Soviets freedom of military action below the level of nuclear war.

Third, the Soviet leadership is able to exploit the balance of terror more effectively than the United States can because of the nature of the Soviet political system. Soviet leaders do not feel great pressure from public opinion in formulating their military and foreign policies and calibrating their use of military threats towards their opponents. At the same time, Soviet leaders can manipulate the legitimate fear of nuclear weapons in the populations of the industrial democracies. This asymmetry lets the Soviet leadership reap the political rewards of nuclear parity and leaves them freer to employ military threat or actual military forces.

Fourth, the nature of Third World conflicts and of popular aspirations in the developping countries favors military conflicts and revolutions -- best suited to Soviet purposes -- rather than gradualism and the evolution of democratic institutions that America would prefer. When nuclear parity eliminates the danger of retribution to their own homeland and empire, the Soviets can engage in low-risk and low-cost adventureary upheas to help accelerate the "historical process" in the Third World.

On the other hand, any American attempts to intervene to preserve the status quo are likely to require large-scale involvement at a high cost. The unpopularity of such interventions, the difficulties of making them work, and the danger of their escalation into confrontations with the Soviet Union makes them extremely difficult for the United States to undertake.

So nuclear weapons give the Soviets clear advantages. Of course the present balance of nuclear terror also presents the Soviet Union with terrible dangers. But it would be foolish to ignore the fact that the same balance of terror also gives the Soviet leaders opportunities for global assertiveness that are unprecedented in the entire history of the Soviet Union.

On the other hand, the advantages created for the Soviet Union by the nuclear revolution do not assure the success of Soviet policies. Those advantages may well be balanced by the Soviets' problems: internal weaknesses, difficulties inside the East European empire, and the Soviets' inability to match their military capabilities with the political, economic and cultural power that could produce practical advantages for the U.S.S.R. around the world. The restoration of an effective American-West European alliance could also help frustrate Soviet ambitions.

In the area of strategic weapons, the Soviet leadership's oft-proclaimed desire for arms control and stability is at least as sincere as President Reagan's repeated statements of interest in nuclear arms reduction. But if nuclear weapons were ever abolished or decisively reduced in number, it is the Soviet Union that would really suffer -- militarily, politically and psychologically. This is an important reason why the total elimination of nuclear weapons is an unattainable goal, and why even their substantial reduction will be very difficult to achieve, though we must strive to achieve it.