A SOVIET INVASION of Poland has been an ominous possibility ever since the great strikes and the formation of independent trade unions in Poland last August. Ready to attack instantly on orders from the Kremlin -- as they did in Czechoslovakia in 1968 -- Soviet forces remain poised along Polish borders.

The challenge to communist structures posed by events in Poland over the past seven months has been unprecedented. Senior American and Western European officials believe that only fear of the consequences has thus far kept the Soviets from military retaliation. However, these officials agree that the moment may come -- perhaps soon -- when Moscow will conclude that the only way to restore "socialist order" in Eastern Europe is to invade Poland, no matter what the consequences.

Conversations with U.S., Western and Eastern military and diplomatic European specialists point to two scenarios, one long-range, the other short, either of which could result from a Soviet invasion of Poland.

The short-range scenario assumes a swift and successful operation. Responding to an "invitation" by hard-line Polish communists (the procedure employed in Czechoslovakia and, later, Afghanistan), Soviet airborne divisions would be landed at night at principal Polish airports, moving on to occupy the cities. Simultaneously, Soviet armored divisions, backed by motorized infantry, would invade Poland from Western Russia, Czechoslovakia and East Germany.

Western intelligence estimates are that Moscow has deployed between 500,000 and 1 million men for action in Poland (it took 200,000 troops to subjugate Czechoslovakia without any fighting).

Central to this assumption is that the 300,000-man Polish army would not fight, and that the KGB (the Soviet secret police) would act in concert with Polish security services to round up the leadership of Solidarity (the Polish workers' movement), independent student and farmer organizations, and KOR (the intellectual opposition group) throughout the country.

Current leaders of the Polish Communist Party and government would be removed, replaced by those who had issued the "invitation" for the invasion. While there would be some strikes and scattered resistance by workers in factories, order would be promptly restored -- even if Soviet or Polish security forces had to use violence.

In such a situation, people would be forced to return to work quickly in order to earn wages -- and eat. This, of course, would be the ideal state of affairs for the Russians once they decide to invade.

Specialists on Soviet affairs doubt that the Soviet population at large would be particularly disturbed by an invasion of Poland. The Soviet dissident movement has been virtually destroyed and, by and large, there isn't much sympathy for the Poles in Russia, a historical phenomenon reciprocated by the Poles.

Besides, as in the case of Afghanistan, Soviet internal propaganda would have convinced Soviet populations that "fraternal asistance" to Poland is really in everybody's interest. It is likewise doubtful that the East Germans or the Czechs would be too upset over an intervention in Poland; many Eastern Europeans resent communist rule, but few of them would welcome an upheaval in the region at this time, and they would not wish to be victimized by Polish events.

In international terms, the consequences of an invasion would be considerable. The Soviet Union's political and economic links with the West, notably the United States, would be severely damaged, although no military response is contemplated. The Reagan administration, already highly critical of Soviet behavior in the world, and demanding that the Russians meet an international "behavior code," would be certain to defer indefinitely negotiations with the Soviet Union on strategic arms limitation (SALT) and practically every other issue. The arms race would be accelerated; Soviet-American challenges and the danger of a general war might increase.

Some American officials believe an invasion of Poland would reinforce the NATO alliance, but this is probably an unrealistic view. Though NATO -- and most members of the United Nations -- would condem the invasion in the strongest terms, the bark would probably be worse than the bite. The United States, Britain, France and several other powers might recall their ambassadors from Moscow and deprive the Russians of badly needed industrial credits, but the West German reaction would be much blander.

Soviet and Eastern European markets are vital for the West German economy, suffering for the first time since the war from substantial unemployment and worrisome inflation, and Bonn might be reluctant to apply economic reprisals against the communist bloc. Also, West Germany is determined to maintain ties with the 17 million East Germans and to protect free access to West Berlin. An invasion of Poland might temporarily interrupt the relations between the West Germans and their communist trade partners, but probably not for long.

On balance,then, the Soviet Union might not fare too badly under the short-range "scenario" of a Polish invasion -- unless a resulting cold war with the United States got out of hand. But it is by no means certain that this scenario would actually work for Moscow.

The Russians could face a very different situation, the long-range scenario.

Such a situation -- and many observers think it is the most likely one -- would result from active resistance by the Poles against any military intervention.

Resistance could take many forms. First, Poles might interfere with the landing of Soviet airborne troops by blocking runways at Polish airports. For an invasion to succeed, Warsaw and other key cities would have to be occupied as rapidly as possible; the Soviet doctrine calls for the use of airborne troops to secure the urban centers as a spearhead for advancing ground forces (this was done in Czechoslovakia in 1968). The Poles, as members of the Warsaw Pact Soviet military alliance, are, of course, familiar with this strategy.

Inasmuch as the Polish air force controls most of the radar installations over Poland's air space, early warning is possible, and the runways could be closed by the placing of trucks, fuel drums and other obstacles at the approach of Soviet transport aircraft. This alone could play havoc with Soviet military planning. The surmise here is that Polish air controllers would be able to act in time. In Prague, infiltrated advanced units of the KGB were able to seize all the communications before the start of the airlift, and the Poles surely have learned the lesson.

A crucial element would be the attitude of Polish security forces toward their KGB colleagues, but it is more than conceivable that they would turn against them. Blocking of runways could be ordered on top-command level, or organized by airbase commanders and individual units acting on their own.

The great unknown is the response of the Polish army, the best in Eastern Europe. It is also the largest Eastern European army in the Warsaw Pact alliance, equipped with modern armor and artillery, and supported by a jet fighter-and-bomber air force. The Polish navy has destroyers as wel as superb Polish-designed Komar missile boats that could oppose Soviet landings on the Baltic Coast. NATO experts rate the Polish armed forces very highly in terms of organization, effectivenes and motivation.

The Polish armed forces are under strict Soviet discipline, but this discipline could evaporate if patriotism overcame ideology. Nobody has ventured to predict whether Gen. Wojciech Jaruzelski, the new premier, and Stanislaw Kania, the first secretary of the Polish Communist Party (the two men are personal friends and close political allies, both favoring the reform movement in Poland), would actually order Polish troops to fight Soviet invaders. But it is likely that even in the absence of such orders (and even if resistance is forbidden), individual troop commanders would take matters into their own hands and resist the Soviets. In the 1956 crisis, the commander of Polish armored forces deployed his tanks to defend Warsaw in the event of an anticipated Soviet invasion, and this could happen again.

If the Polish armed forces are neutralized as a whole -- a Soviet attack is expected to come from the Soviet Union in the east, Czechoslovakia in the south, East Germany in the west, and from the Baltic Sea in the north, literally encircling Poland -- individual units of varying sizes might engage in guerrilla warfare just as the Polish underground fought the Nazis during the occupation in World War II. Detachments may operate from dense forests, harassing Soviet forces, severing communication lines, and forcing the Soviets to act as a full-time occupation army. If such an underground operation were to come into being at an effective level, the supply of arms from the West through clandestine channels might not be ruled out.

At the same time -- and this may be the most important of all -- Solidarity workers (over 10 million Poles, nearly one-third of the population, now belong to the free unions) might lock themselves in their factories, turning them into fortresses. Solidarity leaders were prepared to do this last August during the initial wave of great strikes, and they are ready to order this kind of urban resistance in the event of an invasion.

There are believed to be aresenals of small arms stashed away in many of the factories, and the workers would unquestionably use them if attacked by Polish security forces (if the latter would obey orders to storm the factories, which is doubtful) or Soviet troops. In such an event, the Russians would face the excruciating task of laying military siege to the workers.

The strongly anti-Soviet population would overwhelmingly support the workers by endeavoring to supply the besieged factories with food and other necessities, and even challenging Soviet troops in the streets. Tens of thousands of students would be part of such resistance. And the powerful Roman Catholic Church would actively support and encourage it. Thus the Soviets would find themselves engaged in a bloody battle with the entire country.

The Soviets could also face a lengthy campaign of civil disobedience even after all active resistance had ended. There could be strikes, slowdowns and industrial and military sabotage, forcing Moscow into enormous economic investments to keep the country going.

Given the vastly superior Soviet military might, Poland could be occupied within a relatively short time. But if tens of thousands of Poles were killed in defending their country, the worldwide reaction would, predictably, be one of great severity. A war in the heart of Europe would be quite different from a war in Afghanistan, and the West would respond accordingly.

Militarily, then, an invasion of Poland would be extremely onerous to the Soviet Union, forcing it to maintain an occupation army there. With Moscow engaged for well over a year in a war in Afghanistan, which has tied down hundreds of thousands of Soviet troops through continuing rotation (there are some 90,000 of them in Afghanistan at any one time), and with the need for maintaining over 2 million troops on the Chinese border, a military operation in Poland would weaken the Soviet Union to an unpredictable degree.

Politically, a slaughter in Poland would mean the end of all contacts with the West for years, a state of affairs the Soviet economy can ill afford. Even West Germany, observers say, would be unable to counduct business as usual with the Soviet bloc in the event of a Polish bloodbath. European -- and German -- public opinion would not stand for it. The loss of Soviet influence in the Third World would be incalculabe.

In superpower terms, a warlike situation in Poland would destabilized all the East-West relations in the most basic fashion. It could lead to a much more intense cold war than that of the 1950s. Given tensions with the Soviet Union elsweshere in the world, a war in Poland could push the United States to the brink of conflict with the Russians. It would be, unquestionably, a world crisis of vast dimensions.

Which of these two scenarios has the greatest chance of developing? Clearly, not even the Soviets have a ready answer.