Soviet leaders now evidently consider Afghanistan an integral part of the Soviet Union's military zone on its southern borders, and foresee a virtual state of occupation there for many years to come, with Soviets handling the essentials of government for Babrak Karmal, the man they helped install with a military coup last week.

That assessment of Soviet perceptions and intentions emerges from interviews here with Soviet and well-informed veteran Western sources, who say the Soviets appear to have calculated, and accepted in advance, the substantial losses of troops and material involved in their actions in Afghanistan.

During weeks of intensive study leading up to last Thursday's invasion of Afghanstan, the Soviet Communist Party leadership is said to have reached fundamentally pessimistic conclusions about its complex relations with the United States and China that spurred the leaders to act, even though they knew it might trigger harsh world reaction.

Observers here feel the Soviets intend to convert Afghanistan into another Mongolia -- which is a totally subservient, Soviet-garrisoned state set up by Mongolian Communist in 1921 to help secure the Chinese border.

According to the sources interviewed for this article, there are two basic Kremlin conclusions that may affect East-West global relationships long after the Afghan takeover is a matter for historians.

These conclusions are:

The SALT II treaty, long hailed here as the cornerstone of East-West detente, is dead, killed by a truculent U.S. Senate and hardening anti-Soviet opinion in the United States that will mean tough new military building programs in the coming years on the part of the West and a heightened atmosphere of tension.

Despite Soviet hopes for rapproachement with archrival China after the death of Mao-Tse-tung in 1976, the post-Mao Chinese leadership shows no sign of changing its anti-Moscow antagonism. Peking is said to have rebuffed both public and private Soviet initiatives for relaxing tensions, thus insuring a continued direct threat to the Soviet Union across their heavily fortified border.

This account of Kremlin thinking surrounding the unprecendented invasion of Afghanistan cannot be said to be complete. Many matters of concern to Western policy-makers are unanswered No official Soviet source with direct access to the leadership processes was interviewed. They have been unavailable, in part because of the New Year holiday.

Within the larger framework of traditional Soviet border fears, three basic situations are said to have coincided to bring Moscow to the point of intervention. Not necessarily in order of magnitude, these were:

The fundamentalist Islamic revival taking place in Turkey, Iran and Pakistan (not a direct Soviet neighbor) threatens endless political-religious turmoil among neighbors with whom the Soviets have always sought stable relations. This upheaval is judged as essentially beyond Moscow's ability to control.

But it could lead to the eventual establishment of Islamic governments actively antagonistic to the atheistic Soviet regime, fomenting rebellious or separatist impulses in its own Moslem peoples, who number about 50 million and will constitute the chief source of fresh Soviet manpower through the end of the century.

The U.S. position in Iran can never get worse than it is right now. From a Soviet point of view, if the embassy occupation ends in tragedy, the Soviet leadership anticipates some form of U.S. military retaliation that surely would bring recrimination from many Middle East and Moslem states. This could ultimately establish a direct U.S. military role in Iran with obvious consequences for Soviet border concerns and its own expansionist aims in the Persian Gulf.

If the Tehran crisis is resolved peacefully, then the Soviets fear renewed U.S. influence in Iran despite Ayatollah Ruhollah Khomeini's pugnacious anti-American stance.Khomeini has proved intractable and inflammatory towards Moscow, despite Soviet efforts to curry his favor. The Kremlin is thought to have concluded basic improvements in its government cannot be expected.

Washington, seen from here as fully tied down dealing with its Tehran crisis, could not effectively mount any countermove to the swift, surgical Afghan takeover the Soviets were planning.

Meanwhile, with the rest of the region in turmoil, the Marxist regime in Afghanistan was seen from here to be in critical danger of collapse at the hands of the Moslem insurgents, despite heavy Soviet military and economic assistance. Moscow's choice, Nur Mohammed Taraki, had been deposed and slain in the coup by Hafizullah Amin last September. Amin proceeded to bridle at Moscow directives and further alienate the Moslem guerrillas. Such a reversal, with possible estiblishment of new Moslem leadership in Kabul, was an intolerable prospect for Moscow.

These situations, combined with Moscow's view that SALT II ratification was no longer possible, is seen here as explaining why the Kremlin launched its military intervention last week, instead of waiting for a Senate vote on Salt. If the Kremlin believed ratification likely, it is said here, the Soviets would have waited until the vote was safely in hand and then gone into Afghanistan.

Moscow is said to have concluded from the recent NATO decision on nuclear arms modernization, President Jimmy Carter's higher arms budget, and U.S. formation of a Persian Gulf military force, that detente was at a halt. The Kremlin blames anti-Soviet forces at work in the United States, not its own actions.

It is consistent with this perspective that the Soviets, acting from their little-understood border fears coupled with a will to be a great power, did not foresee or plan for the intense world outrage and apprehension its invasion has caused. Content that they have achieved their goal of controlling Afghanistan, the Soviets will label all foreign denunciations as attempts at strengthening U.S imperialism.

The Soviet leaders are said to see in Afghanistan a new strategic buffer against China, which had strenthened itself by improving relations with the West. The new Afghanistan also is seen as a powerful South Asian wedge with which to exert influence on the other countries of the region, perhaps dampening pan-Islamic tendencies and easing Soviet border fears.

This view, has already been reinforced here by the equivocal stance of India on the intervention, which has more Moslems than Pakistan. Indira Gandhi, seen by Moscow as the likely new Indian prime minister, has favored the Soviets against the United States, China and Pakistan in the past and has not condemned the invasion.

The Kremlin is said to intend to ignore the Western outcry and to blunt the Moslem world's rage by trying to build a hybrid Islamic-socialist state. Karmal will form a national-front government composed of various special interest groups, and find Islamic holy men to speak out in support.

Already today, the official Soviet Tass agency quoted an Afghan Moslem religious leader, Abdul Aziz Seddiq, as expressing "profound joy over the removal of the Amin tyranny that served as a tool in the plans of U.S. imperialism." and thanking the Soviets for intervening.

Karmal will move very slowly in charting fundamental changes in the society likely to cause Moslem backlash, such as full emancipation for women. The Soviets think they have learned well the lessons of their own Basmachi revolution, that convulsed central Asia in the 1920s, as the new Soviet government sought to exterminate Moslem beliefs and traditional life styles.