"There are many reasons why the Russians don't want to hit Poland, and only one reason why they might," a Yugoslav, long resident here in Moscow, said the other day. Rightly.

But the many reasons are secondary. The one reason, if it comes into play, is overwhelming. So, despite the recent calm in Poland, nobody can yet rule out Soviet intervention in the days ahead.

The strongest case against any overt Soviet move springs from the question:

"Then what?" After an invasion, Moscow would have on its hands 35 million hositle Poles living in an economy that is a national disaster. The Russians would be obliged to find food and work for the Poles. But already Soviet assistance to Poland elicits grumbles from Russians who find life none too easy. A current joke here, for example, defines international solidarity as the condition of Russians lining up for food while Polish farmers go on strike.

Even militarily, moreover, occupation would not be a piece of cake. Russia has nearly 100,000 men in Afghanistan. There is a Soviet commitment to protect Vietnam against a resumption of hostilities by China. Soviet troops are poised to take advantage of openings that may develop around the Persian Gulf. Over 400,000 more men are now positioned around Poland. Moving them into action, in these conditions, would put a true strain on the Soviet miliary machine.

Other East European countries -- even those nervous about the spread of troubles from Poland -- are worried by the precedent of yet another Soviet intervention. Thus the declaration of the Warsaw Pact leaders who met in Moscow on Dec. 5 and 6 differed markedly from the joint communique put out in 1968 just before the invasion of Czechoslovakia. Where the 1968 communigue spoke of the obligation of the bloc countries to protect socialism, the Moscow declaration only asserted that socialist Poland "could count on the help" of the Warsaw Pact countries.

That soft approach is all the more appealing because of recent successors accruing to the new Polish leader, Stanislaw Kania. He has dealt with strikes, registered the independent trade unions and passed through the high drama of a mass meeting honoring workers killed a dedade ago by Communist police. He has won the support of the Catholic Church, which, in turn, is now using its influence to enforce discipline on the independent trade unions.

To be sure, Kania may have to pay a price for rapport with the church and the unions. But the Russians claim that they don't care all that much about the mere forms of socialism. As one Communist Party official here put it: "We have a Korean form of socialism, a Vietnamese form, a Yugoslav form, a Cuban form and a Hungarian form. Why not a Polish form?"

Ironically, that flexibility finds a kind of reinforcement in the industralized countries. The Russians understand very well that the United States, France, Japan, West Germany and Britian would react sharply to any military move against Poland. At least for awhile, and maybe even for a long while, serious sanctions would be organized against Moscow. With food in short supply, and with high-technology items badly needed, a new freeze on trade would hurt a lot.

Perhaps more important still, the arms control talks, in which President Leonid Brezhnev has invested much of his personal prestige, would finally go by the boards. The Soviet leader's foreign policy would have shown up as a failure one month before what is supposed to be his crowning achievement -- the 26th Party Congress scheduled for the end of Februrary.

But the Russians would risk all those interests, indeed they would even risk World War III, in one set of circumstances. If the Polish party lost control over events, or if there were any threat to the Soviet lines of communication with Eash Germany, the Red Army would move.

One clear sign of that intention is a massive shift of commanding officers pointing in the direction of military operations. Another sign, on a more personal plane, came my way when Soviet authorities arranged an interview for me with an aging journalist, Vladimir Koudryavtsev of Izvestia, whom I had never met nor asked to see. In some embarrassment, I asked him why the interview had been arranged. "Two reasons," he said pointing to his chest. "I fought in Poland in World War I. I fought in Poland in World War II."