The Soviet leadership has reportedly sent a tough letter to Poland that is reminding some worried diplomats here of a similar note on the eve of the 1968 Warsaw Pact invasion of Czechoslovakia.
At this writing, the diplomats' worries remain directionless, the kind of feeling you get from bumping into an armchair in a dark room.
No one is predicting outright that the Soviets will intervene directly in the Polish crisis. Even if such a decision were taken, most Soviet officials might well be in the dark. Certainly most foreign diplomats would be.
The feeling that does seem to be growing among foreign experts here is that some direct move, even a military one, cannot realistically be ruled out.
On one point there is virtual unanimity: that Soviet concern over events in Poland has heightened sharply, especially with the approach of a special congress of the Polish Communist Party that is likely to enshrine radical reform in that strategically central Warsaw Pact state.
East European sources, generally better informed on the Polish situation and on Soviet thinking than Westerners here, also speak out such a hardening of attitudes.
The key question -- an open one since the Polish labor crisis erupted last year -- is how, or whether, the Soviets will act on these attitudes. Answering it is complicated by the fact that most public Soviet comment so far could just as easily mean either of two things: that Moscow is getting ready to act, or that it wants Poland's battered Communist leadership to think it is.
The same goes for the latest element in the Polish puzzle: reports from Warsaw June 8 that the Communist leadership there had received a sternly worded note from Moscow urging a tougher stand against alleged "counterrevolution" in Poland.
One report said the letter had accused Poland's Communists of breaking promises made to Moscow, presumably pledging such a crackdown. Although the Soviet news media did not comment immediately, more than a few Moscow diplomats found the report particularly unsettling.
Thus far, the Soviets have relied in large part on their official press to broadcast alarm over events in Poland. For foreign observers here, this has become almost background noise, punctuated by occasional "firsts," new areas of evident Soviet concern occasionally reminiscent of coverage of Czechoslovakia in 1968.
A letter addressed directly to the Polish Communist leadership would represent a far more muscular call to order.
Both Western and East European sources noted that reports on this particular here letter recalled similar communications before Soviet military intervention in Hungary in 1956, and in Czechoslovakia 12 years later.
Only days before the August 1968 intervention, the Soviet leadership sent a note to party chieftains in Prague accusing them of a breach of confidence, of breaking promises allegedly given to their Warsaw Pact allies.
Czechoslovakia in 1968 is not Poland in 1981. In Prague, reform came from the party, from above. In Poland, popular pressure has forced change. In 1968, the Soviet- led troop move was accomplished relatively quickly, without major snags.
This, at the very least, can not be taken for granted in Poland.
Moreover, direct Soviet intervention certainly wouldn't help Poland's disjointed economy. The effect might well be the opposite. Western bankers probably wouldn't help out. The Soviets, who are said to have pumped some $4 billion into Poland since the current crisis began, would be in for even higher stakes.
Also at stake, much more than in "predetente" 1968, are significant economic links between the USSR and Western Europe.
The Soviets know all this. There is no genetic impulse among Kremlin leaders that makes them hanker for a good East European invasion every 12 years or so. Senior officials here have made it clear that they would much prefer to see the Polish leadership sort out the crisis for itself.
The Kremlin leaders may not be nice guys; that's presumably not how they got to be Kremlin leaders. But they are not political novices, either.
Should Soviet tanks ultimately move, the consensus presumption here goes, the Kremlin will have figured that the potential losses from intervening underweigh the potential losses from not doing so.
Among the fears most foreign analysts attribute to the Kremlin are:
* That the people who run Poland, an important piece in the Warsaw Pact alliance, are fast becoming unreliable allies.
* That Polish reform, mocking the claims of Soviet theoreticians that the communist party must play the "leading role" in a communist state, could ultimately serve to loosen the East European bloc.
* That the reforms could conceivably prove contagious, not so much in the Soviet Union as in other East European states.
Besides, would the Western countersting be as "fatal" to "detente" as Western words imply?
History (Hungary, Czechoslovakia, Afghanistan) suggests may be not. And so, privately, do some West European diplomats.