FIFTEEN YEARS AGO today, Soviet and Warsaw Pact forces invaded Czechoslovakia to put an end to that country's brief experiment in "Marxism with a human face." Today Czechoslovakia is once again a docile, orthodox member of hat the Russians refer to as their "camp." Virtually all traces of what we called "the Prague Spring" have been wiped away.

But on this anniversary of that event, just next door in Poland there is compelling evidence that defiance in Eastern Europe lives on. This month also marks another anniversary -- the third birthday of the Polish trade union Solidarity. To be sure, a crackdown encouraged by the Soviets has wiped out Solidarity, at least formally, but that operation has not been a success. The spirit of the Polish nation is decidedly unbroken. The country is ruled by a shaky regime that has been unable to restore the political stability that Moscow wants.

New evidence of Poland's instability came as recently as last Sunday, when police in Gdansk broke up a demonstration of thousands of people commemorating Solidarity's third birthday.

So if the news from Czechoslovakia remains grim, the situation in Poland is a reminder that times do change, and that the desire for liberalization in the European countries Joseph Stalin occupied after World War II remains powerful.

Any comparison of Czechsolovakia then and Poland now has to begin with the acknowledgement that the two upheavals occurred in very different international contexts. From the viewpoint of Prague's reformers, 1968 was an inauspicious time to launch independent changes in the communist system, particularly compared to the climate in which Solidarity was born a dozen years later.

In 1968, there had still been no hint of detente -- no Berlin agreement, no strategic arms talks, no real East-West relationship compared to what was about to begin. In the years that followed the web of agreements and trading relationships that have now radically increased East-West interdependence have changed the diplomatic environment fundamentally. American grain sales to the Russians, strategic agreements on nuclear arms limitations, large-scale East European borrowing from Western banks and large-scale Soviet purchases of western equipment and technology (and sales of natural resources) created new realities. In short, in 1968 there were few concrete penalties that the West could impose on the Soviet Union for its actions in Eastern Europe.

By 1980, the stakes for the Kremlin in maintaining its relationship with the West had increased enormously. The Soviet Union and its satellites both had to weigh potentially serious negative consequences if they once again used brute military force to crush a dissident satellite.

But beyond that important difference in the international context, there were fundamental dissimilarities in the Polish and Czechoslovak reform movements that help explain their contrary outcomes and suggest why there is still hope for Polish liberalization.

From the start, the roots of the struggle in the two countries were different. Those of us who covered the Prague Spring were moved by its drama and impressed by the courage and idealism of the communist politicians and intellectuals who inspired it. Yet it was not a national movement to the same extent that Solidarity was -- and is.

Many trace the origins of the Prague Spring to the 1967 conference of the Czech Writers Union, when intellectuals rose to denounce hard-line policies and demand reforms. It was no accident that the first Soviet tanks to enter the capital on the night of Aug. 20-21, 1968, surrounded the offices of the writers' and journalists' unions.

The reforms in Czechoslovakia were primarily the work of communist intellectuals -- philosophers from Charles University, historians, novelists, poets, artists, movie- makers and journalists -- who convinced the new party leadership that the time had come to modernize and humanize the practice of Marxism, and endow it with a dimension of public liberty.

Several of the new Czechoslovak politburo members forcefully embraced these ideas, and they succeeded in bringing along the party's new first secretary, Alexander Dubcek, a Moscow-educated apparatchik who had won his post in a compromise between communist liberals and conservatives, and who probably never entirely comprehended the role that history was preparing for him.

What made the Prague Spring fatally vulnerable, in my opinion, was that the leaders never commanded a unified national following. Most notably, the working class remained largely apathetic.

I remember accompanying Dubcek and his politburo colleagues on visits to industrial plants across Czechoslovakia in the months before the invasion. While the reception was generally warm and friendly, I had the impression that the workers were treating the humanitarian concepts expressed in the speeches as abstractions rather than realities.

It is possible that the workers would have ultimately rallied around the new flag -- especially if the economic reforms being discussed in Prague that spring had been launched. The Russians never gave the Czechoslovak experiment a chance to prove itself, and in the 15 years that followed, the working class simply turned off politically. Communist intellectuals became isolated once again.

Under these conditions, the resurgent Czechoslovak secret police had no serious difficulty turning the country once more into an extremely repressive dictatorship.

In the first year of the Soviet occupation of Czechoslovakia, there were only two major instances of public rebellion against it: a massive turnout in January 1969 at the funeral of Jan Palach, a student who set himself on fire to protest the invasion, anddstreet demonstrations when Czechoslovakia beat the Soviet Union in the finals of an ice hockey championship.

The Stalinist night that subsequently descended on the country was relieved only by occasional flashes of protest, almost all of it from intellectuals rather than workers. Many intellectuals emigrated to escape imprisonment and work in freedom. Others who remained at home were forced to make a living as night watchmen and clerks, and forbidden to publish.

Why, by contrast, is the spirit of protest still alive in Poland, and why does the Polish regime seem so vulnerable in comparison to the repressive dictatorship in Prague?

For one thing, the Czechoslovak reformers lacked the powerful ally that was later to play such a prominent part in the rise of Solidarity: the Catholic Church. Czech and Slovak churches, Roman Catholic as well as Protestant, never played a political role even remotely comparable to that of the Catholic church in Poland.

The Prague regime has had fewer compunctions about cracking down on political activism by the clergy. Priests in Czechoslovakia are state employes, a status that makes them far more vulnerable to official discipline than in Poland, where the church has been able to win a broader measure of independence. In 1983, when the Prague regime tightened up again after some sporadic incidents of defiance, several Catholic priests were arrested for circulating underground religious literature and making contact with dissident intellectuals. Others were defrocked.

The election of a Polish pope and his triumphal visit to Poland in 1979 set in motion forces so powerful they could not be controlled. What occurred in Poland during the 18- month existence of Solidarity was a national movement far different from the Prague Spring.

Solidarity, of course, had broad support in the urban and rural working classes. But it also bridged a gulf between workers and intellectuals that had divided them in previous moments of protest in 1956, 1968 and 1970.

Advisers drawn from the Polish intellectual establishment played a key role in the formation of Solidarity in Gdansk in July and August of 1980, and in subsequent negotiations between the communist regime and the independent union.

This made Solidarity a movement without precedent in the history of the Soviet bloc. It succeeded in tapping Polish nationalism, traditional anti-Russian sentiment and disenchantment with the corrupt and inefficient communist system of government. And, of course, the intensely patriotic and politically-minded Polish Catholic Church helped to coalesce this national revolt against the communist status quo.

Solidarity, like the Prague Spring, was not acceptable to the Kremlin. It threatened the primacy of the Communist Party and was an infection that could have spread. Just how dangerous Solidarity was was evident from the fact that the Polish turmoil exerted a disturbing influence on Soviet Estonia. A senior Soviet official disclosed recently that there were organized attempts in the fall and winter of 1981 to stage strikes emulating those of Solidarity. Authorities had to mobilize all their resources to put down the trouble.

But given the character of the Polish revolt and the new international realities, it was probably clear to the Soviets that Poland could not be "normalized" by the same ruthless measures employed in Czechoslovakia in 1968.

The Kremlin feared that an invasion of Poland would meet with armed resistance. (In 1968, the Czechs and Slovaks accepted their fate passively.) In Warsaw in 1981, I was told on the highest authority that top Polish leaders conveyed a warning to that effect to the late Mikhail Suslov, the Soviet politburo's chief ideologue, who had been dispatched to assess the situation first hand. Soviet military leaders, busy with the military occupation of Afghanistan, clearly did not relish dealing simultaneously with a military action of unpredictable consequences in Eastern Europe.

The long-prepared Polish military crackdown in December 1981 was, therefore, the only possible solution under the circumstances. But it has not worked. Poland has not returned to normal. Gen. Wojciech Jaruzelski, the premier and first secretary, acknowledged to me in May 1982 that the majority of Poles are "not with us," and that his task, if it ever could be achieved, was to bring about national "reconciliation." He never uttered the word "normalization," the Soviets' oft-mentioned goal in post- invasion Czechoslovakia. Normalization meant the restoration of Soviet- style orthodoxy.

Though the Jaruzelski regime has done away with every independent organization in the country, it tolerated more open defiance during the martial law period than could have been foreseen. Even before the lifting of martial law in July, there were street demonstrations, clandestine Solidarity radio broadcasts and widespread distribuition of anti-government leaflets and even newspapers. The regime acted with relative restraint under the circumstances. The security forces used firearms only a few times. As a senior official told me in a private conversation, "We must be careful not to overdo repression because we know that millions of Poles are against us and we can't risk precipitating an emotional explosion we couldn't control without killing lots of people."

"We cannot afford to paralyze the country economically," he added.

That normalization cannot be imposed in Poland is an enormous dilemma for the Warsaw leadership -- and for the Russians. Since last year, the government has released thousands of interned Solidarity leaders, including Lech Walesa, though it still holds about 700 political prisoners, according to official figures.

The regime has failed utterly to turn Walesa into a non-person, and he remains a symbol of Solidarity's durability. He is free to be filmed, photographed and interviewed by foreign newsmen. The government has not imposed censorship on foreign journalists, and clashes between demonstrators and riot police are regularly seen on Western television.

John Paul II's second visit in June was further evidence that the regime is neither willing nor able to dampen the fires of opposition. His meeting with Walesa, clearly the communist government's public enemy number one, showed that even more clearly. Even though photographs of the meeting have not been released, every Pole knows about it and understands its significance: the regime recognized Walesa's continuing political importance.

By incorporating martial law provisions into legislation, the government has the official means of enforcing order -- and repression -- at any given time. But it cannot do away with the sentiments of the population, and its multifarious forms of expression, and so it cannot yet "normalize" Poland. The basic reality is that Solidarity has not been destroyed as a broad national movement.

Polish intellectuals are playing a significant role in keeping the spirit alive. Contacts between intellectuals and workers are being maintained, and in almost every sense Warsaw is a far freer city than Prague.

There the government deals forcefully with the smallest protest. Last June, in one of the few shows of public defiance since the Prague Spring, some 300 youths raced through the streets of the capital shouting, "We want freedom." Police moved in and six were arrested.

Such demonstrations are rare. It is still mainly the intellectuals who keep alive a tiny spark of freedom in Czechoslovakia. Ludvik Vaculik, author of the "Two Thousand Words" manifesto that articulated the ideals of the Prague Spring in 1968, is among those writing underground, courting imprisonment. So is Jiri Hajek, the former foreign minister, who produced an ironic account of secret police efforts to prevent him from meeting a former colleague, the West German foreign minister.

But workers in Czechoslovakia remain politically indifferent. Why they have been so unresponsive to the Polish movement that inspired workers just over the border remains something of a mystery. Whatever the reasons, their political apathy has left the intellectuals of Prague isolated and alone.

But Warsaw is not Prague.

My own belief is that it is in the interest of the West, notably the United States, to help keep the free culture of Poland intact. Unfortunately, Washington does not seem to understand this. Poles complain that the Reagan administration, as well as private U.S. institutions, have kept to a minimum invitations to Polish scholars and intellectuals to visit the United States. There seem to be no funds to send books, technical texts or literature to Poland, and Poles fear this isolation.

"If you Americans send out invitations, we'll find ways of obtaining passports," a Polish friend told me. "If you send books, let us worry how to pry them loose from the post office. Above all, don't let our culture weaken and perish."

This is wise counsel if Americans do not wish to see Poland "normalized" in the manner of its less fortunate Soviet bloc neighbor to the south.