The United States and the Soviet Union are headed into a prolonged struggles during the Reagan administration over the basic terms of their global competition.

At one of the lowest points ever in relations between the two superpowers, the premises of a decade of shattered American-Soviet detente are open to debate. The willingness, therefore, expressed by Soviet leaders and by Reagan advisers to open "discussions," in contrast to "negotiations," in no small distinction. The record on which talks will begin is filled with enough ambiguities, pretensions and accusations to provide endless agrument.

The Soviet Union intent on restoring its original concept of detente. The Reagan administration is determined to overhaul the detente formula drastically, or to abandon it and substitute stiffer terms for any superpower equilibrium.

As it takes that road, the Reagan administration will be hard pressed to sustain allied unity. The Western European nations, living in the bull's-eye of a potential war, are heavily invested, economically, and politically, in a more ambiguous East-West detente than the United States ever has been prepared to accept.

With prodding by the Carter administration, the Western allies stand committed to retaliate if the Soviet Union resorts to force to maintain communist control in Poland. But the form of Western political, economic or diplomatic retaliation is unspecified. The Reagan administration is unlikely to receive allied support for its broader demands on Soviet behavior unless the Polish crisis erupts to destroy all prospects for controlling East-West tension.

The new administration's approach to the Soviet Union has been marked out, in rudimentary form, by President-elect Ronald Reagan, drawn from the strategy of Henry A. Kissinger -- which was anathema to Reaganites when detente was flourishing. It is a course to which Reagan's nominee for secretary of state, Alexander M. Haig Jr., has been committed since his years as Kissinger's deputy in the Nixon White House: to "link" any agreement that the Soviet Union seeks to limitations on the projection of Soviet military power.

The "linkage" that the incoming administration intends to employe to constrain the Soviet Union globally, however, is more challenging than the initial Nixon-Kissinger version. It is a product of what Kissinger subsequently has described as "an unprecedented Soviet assault on the international equilibrium" since 1975. That is the year that the superpower detente compact of 1972 began to crumble.

In rejecting the product of seven years of U.S.-Soviet strategic arms limitation talks -- SALT II -- Reagan has served notice that the Soviet "policy of aggression" will be linked to any arms control pact. "You cannot sit there and negotiate arms," he has said, "and pretend that the Soviet Union is not invading Afghanistan."

Few American have notices, however, that Soviet President Leonid I. Brezhnev has made a counterdemand, which links together the Kremlin's objectives in more subtle language.

Completing a strategic arms pact, Brezhnev said in late November, "lies at the heart of international security." Beyond that, he said, the Soviet Union, in its global conduct, is prepared to "cooperate as equal partners if the American side also displays readiness for this."

As innocuous as that call for equality may appear, it is even a more divisive issue between the two nations than the nuclear balance of power. The United States uses the term equality, in American-Soviet discourse, primarily to signify equal security and equal responsibility in maintaining international stability. The Soviet Union means literal "equality": an equal right to shape the world in order -- or to alter it.

No American government ever has been prepared to concede equality in any measure that approaches that interpretation. It goes to the core of the two nations' ideological, political, military and economic rivalry. Nevertheless, Soviet sources emphasize, their nation is determined to press for increasing recognition of equality in any bargaining over constraints on Soviet actions, as Brezhnev's comment foreshadowed.

That is hardly what Reagan had in mind when he insisted during the presidential campaign that if detente is to survive it must become a two-way street. But there is no agreed right-of-wat between the two superpowers, except in an extremely ambiguous set of principles that each has tried to invoke against the other as though they represent a binding code of conduct.

Policymakers on both sides have known, since American-Soviet detente was formally launched in 1972 at the Moscow summit conference between President Nixon and Brezhnev, that they had no solid agreement about what the "code" means. Before entering the White House as President Carter's national security adviser, Zbigniew Brzezinski said:

"The Russians never deceived us about detente. They always said that there would be mixed cooperation and conflict. We were the ones who were talking about a 'generation of peace.'"

Many Americans were led to believe that the Soviet Union was prepared to forgo the use of force to pursue opportunities to change the existing power balance. Many Russians, in turn, were led to believe that the Unite States was prepared to treat the Soviet Union as a full equal in the world.

Both assumptions were hollow. The firm afreements between the two nations were their accords collectively known as SALT I: to curtail antibalistic missile defenses severely, and to limit offensive strategic nuclear weapons.

Superimposed on that compact, at Soviet urging, was a declaration that few Americans have ever read. It holds a commanding position in the Kremlin's concept of detente: "Basic Principles Between the United States of America and the Union of Soviet Socialist Republics."

That document, in addition to its pledges of "equality," expresses the "common determination that in the nuclear age there is no alternative to conducting their mutual relations on the basis of peaceful coexistence." That term, however, has totally different meanings to the two superpowers.

For the United States, "peaceful coexistence" literally means what it says, but "equality" does not. The Soviet position is just the reverse. Outside of the document, in innumerable declarations, the Soviet definition of "peaceful coexistence" explicitly pledges support for "wars of national liberation," and unremmitting ideological struggle against capitalism.

On those two endlessly disputed terms, "peaceful coexistence" and "equality," a senior strategist in the Carter administration recently remarked wryly, "we flummoxed each other."

The United States never intended to rely on the mutually ambiguous "principles" as the cutting edge of its detente strategy. Kissinger, then Nixon's national security adviser, said at the time that "we have no illusions." He said the document reflected "an aspiration and an attitude. . . . "

"We recognize," Kissinger said, "that Soviet ideology still proclaims a considerable hostility to some of our most basic values. We also recognize that if any of these principles is flouted we will not be able to wave a piece of paper and insist that the illegality of the procedure will, in itself, prevent is beging carried out."

Nevertheless, starting with the 1973 Arab-Israeli war, then the United States first charged the Soviet Union with breaching the "code of conduct," both nations often have waved the same "piece of paper" at each other. Many analysts, therefore, interpret the Arab-Israeli clash of 1973 as a glaring failure of detente.

Others disagree. They see in the avoidance of a superpower conflict in that year a heightened sensitivity to the dangers that could spin out of a regional war. But there are clearly few restraints, if any, the Soviet Union when it sees its vital interests endangered.

Despite Kissinger's disclaimer of any "illusions," he wrote in his memoirs that when the United States agreed to the basis principles, "we interpreted" them, among other things, as "a denial of the Brezhnev Doctrine." That was an exceptional expectation, perhaps comparable with Brezhnev's interpretation of "equality."

What the West describes as the Brezhnev Doctrine was a declaration by the Soviet leader that was used to justify the Warsaw Pact nations' military intervention in Czechoslovakia in 1968: "a threat . . . to the cause of socialism" in one country is "a common problem . . .for all socialist countries."

That rationale, stretched into a response to a request from a newly established Marxist regime, was invoked again by the Soviet Union in sending its troops into Afghanistan at the end of 1979. The same stern warning was served on opponents of the orthodox communist order in Poland, by the Warsaw Pact leaders who met in Moscow on Dec. 5: "Poland . . . will remain a socialist state, a firm link in the common family of the countries of socialism."

No American government ever has been prepared to attack that "link directly; to do so means risking World War III. Even if the Carter administration had won a second term, however, it would have been involved in a major reappraisal of its relations with the Soviet Union. U.S-Soviet ties were "cut to the bone" in retaliation for the Soviet plunge into Afghanistan last December, leaving ratification of the SALT pact dangling in suspense.

A Reagan administration, its planners contend, by spurning that pact in its current form, and increasing American military strength, will have maximum leverage for bargaining with the Soviet Union, with the entire relationship of the negotiating table.

Every new president finds, however, that his ability to shape events is considerably less than he had anticipated. And all are encumbered by the record of the past, no matter how much they insist on a clean slate.

Kissinger attributed the failures of detente in the Nixon-Ford years not to any deficiencies in strategy, but primarily to Congress, traumatized by Vietnam and Watergate, intent on slashing presidential power. He could not obtain the bargaining chips to sustain his strategy.

"Linkage" does not rely on force alone to extract concessions in one set of negotiations as the price for agreement in another. The strategy requires a mixture of carrots and sticks. As Kissinger preferred to describe the mixture, it combines "rewards" or "incentives" to the Soviet Union for good behavior; "penalties" for bad behavior.

Congress confounded the Kissinger linkage strategy by denying his intended rewards and penalities equally. In 1974 the principal carrot that Kissinger held out to the Soviet Union disappeared: Congress tied demands for liberalized Soviet emigration to trade and credit benefits for the Soviet Union. The Soviet Union indignatly rejected that legislative demand in early 1975 as gross interference in its internal affairs.

By January 1976, a major Kissinger stick disappeared also. Fearful of being drawn into a new Vietnam-like war, Congress cut off covert American military aid to anti-Marxist forces in Angola, after an unprecedented Soviet airlift and sealift brought thousands of Cuban troops to the support of pro-Marxist forces. Angola fell to Marxist control, giving the Soviet Union and Cuba a new ally in Africa, with Kissinger crying out that detente could not survive "another Angola."

What survived was a progressively deteriorating detente, plagued by what Americans perceived as "more Angolas," stretching from Ethiopia to South Yemen to Afghanistan, interspersed by Soviet support for Vietnam's conquest of Cambodia.

Haig and Kissinger, in testifying on the SALT II accord in 1979, both placed great emphasis on the need to invoke stiffened "linkage" in dealing with the Soviet Union.

Kissinger supported the proposed treaty with reservations; notably, major increases in American defense spending and a commitment to "the linkage between SALT and Soviet geopolitical conduct."

Haig, then the recently retired military commander of the North Atlantic Treaty Organization, called for postponing action on the nuclear pact pending "a firm, unambiguous demonstration of renewed U.S. strength" and a rethinkage of U.S.-Soviet strategy.

"Now if you are going to negotiate such linkage," asked Sen. Jacob K. Javits (R-N.Y.), "what are we prepared to give, general?Do we think the Russians are going to give us everything and just walk out of these places? What are we prepared to give?"

Haig replied that he was not advocating an attempt "to exact retrenchments from the Soviet Union" where the United States failed to block its changes in the status quo. Instead, he said:

". . . I think the time has come for the United States to recognize that it is very self-defeating to proceed in a mindless way in a number of functional areas -- whether they be arms control, credit or technology transfer with the Soviet Union -- while they are simultaneously conducting blatant interventionisms aimed at our vital interests and those of our allies and our traditional friends in the Third World.

". . . It is time for us to pull up our socks and rethink this issue and put the Soviet Union on notice that the kinds of activities we have been facing recently are no longer acceptable. . .

Any specifics of what the incoming administration would be prepared to trade off as "rewards" for greater Soviet restraint, in trade, credits or other potential carrots, have yet to be indicated. Only "penalties" are explicitly evident as far -- holding SALT II hostage to changes still undefined, with the threat of an open-ended arms race.

A Reagan administration also presumably will intensify the post-Afghanistan projection of American military power abroad, launched by the Carter administration in its commitment to a Rapid Deployment Force in the Persian Gulf. The use of convert American military force, cut off in 1976 in the Angolan episode, undoubtedly will be reconsidered as well.

Four years ago, the Carter administration was on exactly the opposite course on every one of these issues.

During the last presidential transition, the first breach developed in the Carter administration, between Cyrus R. Vance and Brzezinski, on initial bargaining tactics with the Soviet Union.

That early discord never was revealed. In those heady days at Plains, Ga., nothing was allowed to mar the Carter administration's proud image of unity. Even the closest associates of the secretary of state were unaware of his first, portentous dispute with the president's national security adviser.

Brzezinski wanted blunt, private talks in the Kremlin with a senior policymaker, according to informed sources, "to show them, credibly, what the United States will do" in response to Soviet behavior that it finds intolerable. Brzezinski suggested that he conduct the talks.

Vance was totally opposed. The proposal inescapably raised the image of Brzezinski's predecessor, Kissinger, negotiating with Soviet leader Brezhnev over the head of Vance's predecessor, William P. Rogers, who was reduced to a humiliated secretary of state. All members of the Carter administration, including Brzezinski, were pledged against a repetition of that divisive pattern, which Kissinger -- out of office -- deplored.

Policy discussions with Soviet planners, Vance was determined, would be conducted by the secretary of state. Moreover, he intended to pursue his own lawyer's style of low-key negotiations, breaking down problems into their components, to be resolved step-by-step, with the highest priority assigned to completing SALT II.

Limitations on nuclear weapons, members of the Carter administration were agreed, are a matter of mutual survival for the United States and the Soviet Union, not "a favor" to the Russians that could be turned on or off as superpower tensions rise or fall.

The Nixon and Ford administrations held the same position. Kissinger, after Angola, however, publicly added a warning to the Soviet leadership: "At the same time, it must be understood on both sides that if tensions increase over a period of time, the general relationship will deteriorate, and therefore the SALT negotiations will also be affected."

That was essentially the Brzezinski position as well, as it later emerged. Vance, by contrast, was convinced that nuclear controls were so vital that they warranted maximum insulation from all other issues. Carter gave no encouragement to Brzezinski's idea for initiating talks in Moscow; Vance dismissed it from his mind, not realizing that it would reappear as one of the most divisive issues inside the administration.

In one of his first comments as secretary of state, Vance dismissed "linkage" as a basis for U.S. strategy. "No, there is no linkage," he said. ""I think each of these subjects is an important subject and each should be discussed on its own footing."

It was ironic that Vance, contrary to all his own basic philosophy, ended up in resounding collision with the Soviet Union in the first major encounter with the Carter administration in March 1977. Acceding, with private misgivings, to Carter's determination to make "deep cuts" in nuclear force levels, Vance led the disastrous mission to Moscow that attempted to leapfrog the negotiating pattern of the Nixon-Ford years. The Soviet Union, doubly stung by a sharp departure in the terms for a nuclear accord combined with the Carter administration's open championship of "human rights" for Soviet dissidents, summarily rejected the Carter proposals.

The Carter administration never fully recovered from that initial miscalculation. By the time the nuclear negotiations were restored to manageable terms, the Carter administration was under relentless attack from American "hawks" for "caving in" to Soviet demands, with SALT the target for every Soviet action that conflicted with American interests.

Caught in that cross fire, the Carter administration, divided internally over whether to impose "linkage" which Vance disclaimed, split openly in early 1978.

The issue was the entry of thousands of Cuban troops, Soviet military advisers, tanks and jet fighters into Ethiopia starting in late 1977, to repel the attempt by Somalia to regain control of the disputed Ogaden region of Ethiopia, inhabited by Somalis. It represented, to Brzezinski and many Americans, "another Angola," but this time with greater Soviet involvement.

There was one important difference. The Marxist regime in Ethiopia was within its rights, under international law, and also in African practice, in asking support to repel an attempt to alter its borders. At the same time, the Soviet-Cuban presence in Ethiopia raised alarm in the Carter administration that the conflict would spread over the Somali border. The question was how to preclude that possibility. Vance favored quiet diplomacy; Brzezinski took the opposite course.

Directly linking the SALT negotiations to the conflict in the Horn of Africa, Brzezinski publicly said the United States had made "substantial concessions" in the nuclear talks, demonstrating its "political will" to reach an accord. He called on the Soviet Union to demonstrate an equal political desire to halt the Ethiopian-Somali conflict.

"We are not imposing linkages," Brzezinski said, "but linkages may be imposed by unwarranted exploitation of local conflict for larger international purposes."

Carter publicly took a position closer to Brzezinski's than to Vance's. Inside the government, however, it was the Vance position that prevailed. Vance gained private assurances from the Soviet Union that Somali's border would not be crossed, and Brzezinski's plea for dispatching an American naval task force into the region, to demonstrate that the United States had "faced down" the Russians, was rejected.

The argument over which course of action to take in dealing with the Soviet Union, however, left deep marks on American political debate. In a favorite, wry metaphor that Brzezinski put on the public record only after the Reagan election victory, he attributed the collapse of American political support for SALT to the failure to draw a line at an early stage on the extension of Soviet military power, saying: "SALT lies buried in the sand of Ogaden."

What killed the SALT agreement, a senior official in the State Department countered recently, was the attempt to link it "politically to the Horn of Africa," rather than the failure to do so.

Quiet diplomacy had forestalled the only real danger to American interests, said Brian Atwood, assistant secretary of state for congressional affairs. Without naming Brzezinski, Atwood said that by raising "linkage" as a public issue, "the administration had inadvertently legitimized the opponents' most telling argument [against SALT] and had itself undermined the chances for ratification."

"In acknowledging a linkage between Soviet conduct and SALT," Atwood said, "the careful attempt to insulate SALT was over." As a consequence, Atwood maintained, everything that followed in Soviet global conduct rebounded doubly against the administration's prime foreign policy objective. Invoking "linkage" so blatantly in U.S.-Soviet policy, he protested, is an invitation to "extremists in our body politic . . . to preempt" any administration's policies "with a harder line than reality merits" or that any government can deliver.

The incoming administration starts from the opposite premise: the more militant it is perceived to be, the greater its freedom of diplomatic action.