For 35 years after the end of World War II, the leaders of the Soviet Union felt constrained from using their own military forces to fight in any country outside the empire they inherited at war's end. Last month those restraints stopped working.

In the judgment of a cross section of this country's leading Kremlinologists, the Soviet invasion of Afghanistan deserves the overworked adjective "unprecedented." That is what makes it so alarming to the Carter administration and to students of Soviet policy outside the government.

"Either it is a terrible miscalculation," observed Prof. Seweryn Bialer of Columbia, who is just completing a major study of Soviet policy, "or it is a terrible calculation." Either way, Bialer said, "it is a qualitative change in Soviet behavior. There is no doubt in my mind that it is a test case."

Interviews with Kremlinologists across the ideological spectrum reveal some differences of interpretation of the Soviet invastion, but also a broad area of consensus on these principal points:

Though the decision to invade was probably taken out of a sense of desperation about the Soviet position in Afghanistan, a neighboring country, it also reflected Soviet reaction to American policy and the plight of Soviet-American relations.

The Soviet leaders had to realize that they were departing dramatically from a longstanding policy of caution by using their own forces to annex a nation not previously regarded as part of the Soviet empire.

At the same time, the Soviets probably underestimated the alarm their move would cause, both in Washington and in other world capitals. They may have felt that the world's relative indifference to Soviet-Cuban advances in Africa and the fall of the shah of Iran was grounds for optimism about a restrained international reaction to this move.

Though there was probably heated discussion within the leadership about whether to invade Afghanistan, it is unlikely that the decision represented a split between hawks and doves in the Kremlin.More likely, the hard-line feelings that have always typified Soviet leaders won out in this case over the pro-detente sentiments that have also infected the group of old men who run the Soviet Union.

The next generation of Soviet leaders, if not the present one, may well come to view the invasion of Afghanistan as a costly miscalculation.

Interestingly, some of the Kremlinologists who disputed hard-line interpretations of Soviet behavior in the past are among those who view the invasion of Afghanistan as most alarming. Bialer of Columbia, for example, noted that he does "not have a reputation as a hawk."

Another in the same category, Robert Legvold of the Council on Foreign Relations, echoed Bialer's interpretations. By invading, Legvold said, the Soviets "have burned bridges and they knew they were burning bridges. . . . This represents a change in the underlying assumptions of Soviet policy."

Some harder-line analysts tend to regard the move into Afghanistan less starkly. One, Prof. Adam Ulam of Harvard, said he thought the invasion was a rather straightforward extension of recent Soviet behavior. "For the last three or four years they've been led to believe they can get away with anything," Ulam said of the Soviets. The real departure for them was the Soviet airlift of Cuban troops into Angola in 1975, Ulam said.

It may take years to accumulate enough evidence to explain why the Soviets decided to burn those bridges and move into Afghanistan. A full accounting of the relevant factors would depend on information about personalities and processes in the Kremlin that remains inaccessible even to Soviet officials outside the inner circle in Moscow, let alone to foreigners.

The crumbling of Soviet-American detente was certainly one of those factors. Early last month, one of the most persistent optimists about detente in the American government, Marshall Shulman of the State Department, addressed the Council of Foreign Relations in New York. His talk was filled with foreboding. Relations with Moscow were very bad, he said, and they could get worse.

Clearly the Kremlin's Americanologists had come to similar conclusions. They had watched as the SALT II debate became a vehicle for American hawks to win commitments for increased defense spending. They had watched the Europeans and Americans first ignore an offer (made Oct. 1 by Soviet leader Leonid Brezhnev) to negotiate new reductions of forces in Europe, then ignore Soviet threats and decide to deploy in the European theater new rockets that deeply alarmed the Soviets. They had watched the United States move continually closer to China, finally offering Peking the trade that Moscow had long coveted and long been denied.

"They had nothing to fear fro us," Bialer observed, "and nothing to hope for with us either." Henry Kissinger's dream of a web of relations that would restrain the Soviet Union had evaporated. Apart from SALT II, detente held out no carrots to the Soviet Union. And the price they would have to pay for SALT II seemed to grow all the time.

The Soviets may have felt that American and its allies would accept their move into Afghanistan with the same combination of rhetorical protest and inaction that they used when the Soviet-Cuban combination moved into Angola and Ethiopia. Certainly they had no direct experience to suggest strong western action, though it is hard to imagine that they failed to forsee that their move would cause shock in other countries.

Many American analysts believe the Soviets felt immense pressure to preserve a communist regime in Afghanistan. According to this view, the Soviets may have believed they faced a choice between withdrawing and accepting a humiliating victory by the Moslem rebels there, or moving in strongly to seize control. "The armies of socialism march only in one direction," a Soviet observed to an American last week, implying that the Soviets could not let an avowedly Marxist-Leninist regime go down the drain. s

But the argument that somehow the Soviets had to invade Afghanistan FINDS LITTLE SUPPORT AMONG PROFESSIONAL kremlinologists. There is widespread agreement in that fraternity that whatever the pressure they felt, the Soviets had to realize that their action represented an important departure from past behavior.

"If they had been playing under the old rules," Legvold said, "they would have continued as they were going (propping up the Afghan communists as best they could with aid and advisers), or they would have cut and run." Or perhaps even sent in Cubans -- but not invaded.

So what explains the willingness of "these cautious old men," as William Hyland described them, to take this leap? Perhaps, speculated Hyland, an associate of Kissinger's and longtime Kremlinologist inside the government, those old men realized they were nearing "the end of their days" and said, "'it's time to cash in the chips we've been accumulating.'"

Prof. Robert Tucker of Johns Hopkins University offered a similar idea.

"What would you do," Tucker asked, "if you were a rising power with pretension, aspirations," and you saw a favorable moment for bold action? Tucker added that the Soviets must realize that the "correlation of forces" about which they write and talk so much is unusually favorable to them just now, and is likely to turn against them again in the 1980s, thanks to new initiatives undertaken in the West.

Hyland noted that there have been previous occasions when the Soviets decided to "pour it on" in relations with the West. They did so after the U2 incident, and in 1957, after the first Sputnik was launched, the Soviets walked out of the Geneva disarmament negotiations, Hyland recalled.

None of these speculations address directly the question of fundamental Soviet intentions. Kremlinologists have argued for years whether the Soviets seek to dominate the world, to achieve military superiority over the West, or simply to provide maximum possible security for their motherland while exploiting all targets of opportunity they encounter.

Bialer has suggested an answer to this question that may be helpful to amateurs: The Soviets hope to get military superiority and perhaps dominance, but they do not expect to succeed. They have a healthy respect for their own weaknesses and their rivals' strengths.

That thought raises a baffling question about the invasion of Afghanistan. For many years, a cardinal element of Soviet policy has been to avoid moves that would push the Americans into making their best efforts -- in military technology particularly. Though big and powerful, the Soviets do not have the economic base or the technical capability to match the United States in an all-out competition, as the race to the moon suggested. Yet Afghanistan could push America into a best effort, or at least a much better one.

This is one reason many professional students of the Soviet Union regard the move into Afghanistan as a miscalculation. "It was a mistake from their point of view," Ulam said.

"For all the '80s," Bialer predicted, "they have precluded the possibility of being in the military situation of the late '70s. They'll look back at the late '70s as a golden era." The Future

It is clear that the Soviet invasion of Afghanistan is one of those turning points that define the beginnings and ends of eras. In this case it was the era of Soviet-American detente that ended.

But what follows? Tense and difficult times, certainly. The Soviets will likely retreat from their position of unprecedented exposure to the outside world -- one of the accomplishments of the detente era. They may tighten up at home and try to impose some discipline on their satellites, though it is unlikely they could restore pre-'70s discipline in Eastern Europe.

Tucker of Johns Hopkins predicts that if Soviets manage to pacify and control Afghanistan, the recently fashionable conventional wisdom that military power had lost its utility in the modern world will pass quickly out of fashion. In that case, the invasion of Afghanistan could produce a great boom in the arms industry worldwide.

In any case an expensive and potentially dangerous expansion of weaponry now looks likely.

Perhaps the greatest immediate dangers lie in Afghanistan's immediate neighborhood, particularly in Iran. For example, what are the prospects that the Soviets might try to sponsor a communist coup in Tehran a month or a year from now? And if that were to succeed, might the Soviets send troops to help preserve that new Iranian government as they sent them to Afghanistan?

Ulam offers an optimistic view. "If we react strongly now . . . I don't think they'd go into Pakistan or Iran," he said. He predicts a speedy consolidation of the Soviet subjugation of Afghanistan, followed by a massive new Soviet "peace offensive" around the world "to recapture some of detente."

That is a point on which all the experts interviewed for these articles agreed. Sometime soon, if the Afghan dust settles, Moscow will again be trumpeting its earnest desire for peaceful coexistence, detente and peace.