This analyst's reading of the signs from the Kremlin suggests that the Politburo decided on Thursday, Nov. 27, in principle, to intervene in Poland, leaving the date to be determined later.

The immediate spur was the threat of a strike by Warsaw workers to enforce their demand for a reform of the security services, but this was merely the culmination of a week that saw the beginnings of the unraveling of the communist system, first in Poland and then perhaps in the neighboring countries. That, at least, is how it would have appeared to the men in the Kremlin.

For them, the real crisis began with the call for a rail strike, which the Kremlin denounced as a threat to Poland's defense interests. Moscow made it clear that any such threat to the lifeline of the Soviet forces in Germany would not be tolerated. The general rail strike was averted, but the earlier four-hour strike had shown that Polish railway men had the power and the will to act if and when they decided to do so. From then on, Moscow had to reckon that the Poles could cut its most vital military rail link at any moment of their choosing.

As the week progressed, the demand for the release of the Warsaw printer accused of reproducing a secret police document grew into a campaign for the curbing of the secret police itself, and it was once again reinforced by the threat of a general strike. It soon became evident that the reform of the security services demanded by the workers would amount to the virtual elimination of their power. That is how it began in both Hungary and Czechoslovakia, where the secret police were discredited and undermined well before the political reform movement acquired any real strength.

It has often been argued, wrongly in my view, that the real sticking point for Moscow would come when the Poles began to demand or to implement political reform such as the abolition of censorship and the dilution of the Communist Party's "leading" role. But the Kremlin knows that the party's power in Russia and in such countries as Poland rests ultimately on the secret police.

The threat of the morale of the security services implicit in the Polish call for an investigation of the abuses committed by them would have crowd them to the point where they could be no longer effective as a bulwark of the regime against the popular demand for political reform.

Any Soviet intervention would have far greater chances of success before the secret police began to disintegrate. Holding down Poland after an invasion would, again, be much easier if it were done with the help of a secret police apparatus whose self-confidence remained unimpaired.

These were the considerations that would have been foremost in the Soviet leaders' minds as the Kremlin's "political work" -- which culminates every Thursday with a meeting of the Politburo -- was building up to a crescendo. It was clear that the Politburo had made no final decision on Poland before then, but it was equally clear that the various components of the Soviet machine that would play a crucial part in an invasion -- such as the military, security and foreign policy networks -- had to have clear instructions now on whether an invasion was a practical option. They needed, in other words, a decision from the Politburo, at least in principle, in order to begin the complex series of moves necessary before the Soviet Union could mount an invasion.

As the week progressed toward Thursday, the situation in Warsaw became increasingly uncertain and confused, which would have tended to strengthen the position of those Politburo members who wanted a decision in favor of an invasion, if only in principle. Just a few hours before the Politburo was due to meet, during the night from Wednesday to Thursday, the arrested Warsaw printer was released by the police in an attempt to prevent the buildup of passions to the point of an explosion.

To the men in Moscow, this would have looked like the first step down the slippery slope in spite of all the explanations offered privately by Warsaw.

And, indeed, in the course of that Thursday it looked as if this concession had only strengthened the union's determination to insist on the emasculation of the secret police and on the release of avowedly anti-Communist political prisoners. The Warsaw demand was reinforced by the threat of a general strike -- which would include a rail strike -- and was formally supported by a national meeting of Solidarity union representatives. To Moscow, it would have seemed that to delay a decision would be to court disaster.

On top of everything, even the publication of the party newspaper Trybuna Ludu had been delayed by striking printers -- an ominous reminder that the mass media could be turned rapidly against the regime.

On that Thursday, Nov. 27, the Polish situation would have clearly been the main, if not the only, item on the Politburo's agenda. To delay a decision until the situation in Poland had become clearer was to risk that it might get out of hand before the Politburo could meet again for an emergency session. It was safer, from the Kremlin's point of view, to send out a provisional order to get everything ready, in case it might be necessary to move suddenly -- in other words, to take a decision in principle, in favor of an invasion.

The atmosphere in which the decision was taken is shown by Soviet press criticism of the Polish government for favoring the Solidarity unions over the official Communist trade unions. The Soviet press did not at first reprint the Prague warnings that other Warsaw Pact states could take the same action against Poland that they had taken against Czechoslovakia in 1968 -- but it reproduced them during the weekend following the Politburo meeting.

The Kremlin was taking steps to prepare its own population for the possibility of an invasion. The Soviet propaganda machine was swinging into action. New military movements were detected by Western intelligence on Poland's border. The Soviet forces in East Germany closed the border area to Western observers. The military machine was also swinging into action.

Any Soviet decision would also have to be taken in the light of external circumstances, the chief of which is the inauguration of Ronald Reagan on Jan. 20. While an invasion would certainly mean the end of detente for a while -- after Hungary and Czechoslovakia, the freezeup lasted between two and three years -- the Kremlin knows that it needs to move quickly, if it is to move at all. An invasion launched after Jan. 20 would be regarded by Reagan as a personal challenge, and he would never forgive the Russians so long as he remained president. The danger period is the time around Christmas, as the invasion of Afghanistan demonstrated last year.