The best that could have happened in Poland has been put out of the question by the declaration of martial law: a government of national unity including Solidarity, guarantees of civil and political rights, economic decentralization as the nucleus of a new market socialism, membership in the International Monetary Fund and access to Western credits--all this within a continuing alliance with the Soviet Union.

But what has happened is far from the worst. Continued economic deterioration, for instance, could undermine those seeking a moderate course. An accumulation of local incidents over, say, food supplies and the absence of meaningful economic reform could lead to open violence in the streets and the extensive use of force. This would invite the imposition of order by other Warsaw Pact members.

The Soviets would clearly prefer to avoid this option. Occupation would be difficult and expensive. Relationships with West Europe would be seriously damaged. Any hope of an arms or troop reduction agreement with the United States would be suspended. An invasion of Poland would almost certainly widen differences within the world communist movement.

Nevertheless, Poland's geopolitical importance to the Soviets would make all of these considerations secondary. A chaotic Poland, incapable of maintaining domestic order and of fulfilling its Warsaw Treaty commitments, would represent too grave a security risk. Paradoxically, if the decision to invoke martial law has the effect of restoring order in Poland, the possibility that the worst-case scenario would take place may actually diminish.

But there is a third scenario. Politics is the art of the possible, and what seems possible in Poland in time is that the country will "muddle through." Politically, the continued domination of the Communist party is almost certain. There will be no significant increase in political pluralism, at least in the Western sense, or in real political participation by the majority of citizens. Solidarity will not become a competing political party.

However, if the declaration of martial law is temporary, some positive changes seem more likely when things return to normal. Solidarity includes better than one of every four Poles, and this will make a return to the pre-Gdansk status quo impossible. Solidarity's leaders may yet find some direct role in the government. The press will enjoy greater freedom. Civil liberties will be less arbitrarily dealt with. Gen. Wojciech Jaruzelski, whom many Poles see as the last chance, will use his personal prestige to eliminate or at least minimize the strength of hard-liners. He will try to introduce a military- like discipline and merit criteria for recruitment into the bureaucracy.

The recent use of troops to oversee food distribution was generally well received by the population--evidence that the general is seen as relatively even-handed. The arrest of members of the Gierek regime along with Solidarity leaders will probably enhance this impression. Jaruzelski may find the Polish parliament useful in helping to modernize socialism.

Most important, the maintenance of political order in Poland and a continued allegiance to the Warsaw Pact will make the costs of invading Poland greater than any benefit the Soviets might derive from it. The future of Poland will continue to remain in Polish hands as long as the major political actors in Poland moderate their demands in the face of disaster.

To some extent, the outcome depends on policies adopted by the West. Unfortunately, some in policy-making positions seem to think that the worse things get in Poland, the better for the United States. A Soviet invasion would confirm the view that the Soviets are as evil as President Reagan's conservative supporters have always said, and would justify his administration's arms buildup.

That approach ignores what is best for 36 million Poles, and it ignores America's best interests as well. Economically and politically, these interests are best served by a stable Poland. Achieving that goal is vital to the repayment of Poland's huge debts, which a Soviet invasion could nullify, and to the democratization of socialism elsewhere in East Europe. Evolution, not confrontation, is the solution American policy should encourage.