Major decisions in Moscow and Washington in the past few days, and the interplay between them, have cast a dark shadow on international relations in the first year of the 1980s and perhaps for years to come.

The immediate cause of the change in the world outlook is the Soviet intervention in Afghanistan beginning Christmas week, but the immediate effects range far beyond that country and beyond the troubled region of Southwest Asia.

The list of retaliatory measures against the Soviet Union announced by President Carter Friday night is the beginning rather than the end of the drastic consequences. Far more important than reduced grain sales, restricted technology transfers and curtailed fishing rights is the thinking behind these measures in the high councils of the U.S. government.

Carter, who said early last week that the Soviet action had a greater impact on his assessment of the Russians than anything else since he has been in office, was reported yesterday to be both impressed and depressed by the brutality and deception employed by Moscow's forces in Kabul. According to those close to Carter, not only his judgment but also his feelings about the Soviets were affected.

Secretary of State Cyrus R. Vance, who had consistently spoken up for a businesslike and optimistic view of the Russians, is reliably reported to be equally shaken. Vance is said to feel he gave a cooperative relationship with the Soviets his best effort, but that this effort has failed, at least for the foreseeable future.

National security affairs adviser Zbigniew Brzezinski, who has long maintained a darker picture of the Russians and expectations of an era of "overlapping imperial power" between Moscow and Washington, has been confirmed in his views. Brzezinski's plans for intimate relations, verging on a de facto alliance, between Washington and Peking have been embodied in the last-minute instructions to Secretary of Defense Harold Brown, who arrived in the Chinese capital yesterday.

There is no expectation in policy-making circles here that the Russians will respond to western pressure by withdrawing their forces from Afghanistan, where they appear to be positioning themselves for a long haul. Moreover, there is little expectation of a near-term change for the better in Soviet activity elsewhere as international tensions rise, but considerable apprehension about potential changes for the worse.

Without a change in the flow of thinking, which would require a change in the flow of events and perhaps a leadership change in Moscow of Washington or both, the chill in East-West relations is likely to be an international fact of basic importance for many months.

To think or speak of this as a return to the Cold War of earlier decades is deceptive, because the world is more complex now, the margins of safety and patience are thinner, and international life is more difficult to manage.

In short, the world is a more dangerous place, and the new events and attitudes are likely to make it still more dangerous. Among the likely consequences of the new turn in Soviet-American relations are spiraling increases in military budgets and armaments on both sides, greater danger that proxy wars will directly involve great powers, and a greater likelihood of the spread of nuclear weapons to additional nations, some of them highly unstable. The prospect is enough to make one wish for "the good old days" of the 1970s.

All this may be considerably more than the Soviet leadership anticipated in the calculations -- or miscalculations -- leading to the intervention in Afghanistan. There is still little or nothing known in Washington about the details of this decision, which is believed to have been made about a month ago.But on the basis of attitudes and historical analysis, rather than hard facts, there are two schools of thought about the Soviet motivations.

One school of thought is that the Kremlin acted on national security considerations of a largely local character, possibly at the behest of military officials who were committed to "not losing" in Afghanistan. To allow the Marxist revolution there to collapse, as it was in danger of doing, would be to court the spread of a Khomeina-like Islamic revolution to a second nation on Soviet borders. Such a development would be dangerous as well as distasteful in Moscow's eyes.

The second school of thought emphasizes the strategic nature of a Soviet thrust toward the warm waters of the Indian Ocean and the oil-rich Persian Gulf at a time when the United States is engaged in a quarrel with Iran in this view, the Kremlin seized an opportunity to reap long-term advantages. Advocates of this line of thinking speak of Afghanistan as a Soviet "test" of the West.

The two schools of thought overlap to a substantial degree, with a number of officials believing that both fear and opportunity were elements in the Soviet decision. The differences in view about Soviet motivation affect attitudes toward future cooperation with the Russians but both schools agree, with little dissent, that a western response to the Soviet action was imperative.

Both Carter and Vance as well as Brzezinski are said to have decided from the first news of the Soviet-induced Afghanistan coup that it was a major international event with grave consequences. The discussion and debate took eight days because U.S. allies, as well as Third World nations, were involved in the countermeasures.

One countermeasure that generated little debate but holds the seeds of long-term consequences is the decision to extend economic and military assistance to Pakistan, the now-threatened neighbor of Afghanistan. U.S. aid had been cut off last April under nonproliferation laws because Pakistan was discovered to be building a secret uranium enrichment plant capable of producing the raw material for atomic weapons.

By pushing ahead to aid Pakistan despite its continuing nuclear program, the administration is cutting the ground from under its antiproliferation effort. It would be difficult for the United States to penalize India for refusing international safeguards on its nuclear program, even while aiding Pakistan under present circumstances. Carter is reported to be preparing to use his presidential authority to release two controversial shipments of atomic fuel for India following that country's elections this week.

An India-Pakistan nuclear weapons race and the enfeeblement of U.S. antiproliferation efforts could have an effect on other nations on the threshold of atomic weapons capability, including Argentina, Taiwan, and South Korea. To envision the eventual consequences, one can imagine the present situation in Iran and the Persian Gulf if the Islamic revolutionaries had inherited a small stockpile of nuclear weapons from the shah.

On a global rather than a regional level, the consequences of Soviet action in Afghanistan and the U.S. reaction to it are also severe.

In the strategic weapons field, Carter has been forced to shelve his highest priority foreign policy initiative, the SALT II treaty, for at least the time being -- almost certainly for the rest of his elected term.

Even before this action, Carter committed himself to a far higher increase in military spending over the next five years than he had previously scheduled. The Soviets, who are believed to be in the allocation stage of a new budgetary cycle, are expected to jack up their already high military spending programs.

The most fundamental aspect of U.S.-Soviet relations is the fact that each nation has the physical power to obliterate the other militarily. Unbridled military competition, especially in a high-tension atmosphere, can increase the risk that this power may be used, as irrational and unthinkable as that may seem.

With SALT II on the shelf, there is little chance for the other arms control negotiations that Carter initiated so hopefully three years ago. Carter's attempt to restrain the traffic in conventional armaments, for example, is doomed in an era of rising international tension, with or without Soviet cooperation.

The anti-Soviet measures announced by Carter Friday night are intended to penalize Moscow for the intervention in Afghanistan by denying many of the benefits of Soviet-American cooperation in the era of detente. In view of the extensive web of relations between Moscow and other capitals built in the detente era, Washington's actions can only be effective if the high technology nations of Europe and Japan as well as the grain exporting nations of Canada and Australia join the effort.

It is not possible in the 1980s for Washington to rule allies by fiat. At the moment, the allies agree that strong action must be taken in response to the invasion of Afghanistan. Washington officials expect a major drive by the Russians to persuade Western Europe as well as Third World nations that the reaction to events in Kabul is overblown.

It should not be overlooked that national and international tension caused by the long-running crisis over the American hostages in Tehran is the backdrop for the present crisis with the Russians. Whatever its effect on Soviet action, the frustrations and sense of powerlessness in the United States because of the Iranian crisis clearly have added to the intensity of the Washington reaction to Kabul.

In some quarters there seems to be almost a sense of relief to be dealing with a relatively familiar if dangerous East-West confrontation rather than the obscure Iranian puzzle, seemingly so immune to normal pressures and appeals.

Washington officialdom and the public are likely to be drawn back to the Iranian crisis soon. If economic sanctions are imposed on Iran by the United Nations, American hostages may be placed on trial in Tehran in small groups or collectively. Carter may then face a day of reckoning over Iran, with an added dimension of new tension between East and West.