Just how tough will Western Europe get if Soviets move into Poland?

Any Soviet use of troops against Polish workers would force Western Europe into actions it still hopes won't be necessary: Freezing arms-control talks with Moscow, renewing talks with the US on possible boosts to NATO defense spending, suspending diplomatic contacts, and halting trade.

The questions then would be: How deep would the deepfreeze policy turn out to be? And how long would it last?

Much would depend on how far the Soviets go in Poland, how gallant Polish opposition is, and how much blood is shed.

There is little doubt that Soviet tanks in action against Solidarity free trade union workers would have a deep effect in Bonn and London, would stir opinion in France, and be condemned also in the smaller European capitals.

Beyond that, reactions would differ in tone and depth for various reasons.

The West German government could be expected to freeze trade with Moscow. Although Bonn is now the free world's largest trader with the Soviets, and has been so for several years, the trade amounts to only 2.2 percent of West German trade worldwide, and only one-fifth of German trade with the far smaller Netherlands.

The foreign policies of Chancellor Helmut Schmidt and his foreign minister, Hans-Dietrich Genscher, are geared to "ostpolitik" (the window to the East). Sources in Bonn report recognition high in the government that Poland is a special case -- too close for Bonn to press ahead with business as usual if the Soviets invade.

The French, too, are said by sources in Brussels and at the recent Eurosummit in Maastricht, to have been very alarmed by the Soviet invasion of Afghanistan and relentless Soviet arms programs.

While some skeptics see current French closeness with Secretary of State Alexander M. Haig Jr. dictated by President Valery Giscard d'Estaing's desire to impress French voters, other sources see a definite French cooling toward Moscow.

Here in London, Prime Minister Margaret Thatcher has repeatedly called for Soviet withdrawals from Afghanistan. She has warned of the dangers of a Polish invasion. She can be expected to support President Reagan's calls for a diplomatic and political freeze against Moscow if it does invade, and to agree to a suspension of arms-control talks.

So the big three European powers are likely to support Washington on measures toward Moscow -- for the short-term at least. Both France and West Germany would have economic reasons to want to resume normal trade and diplomacy fairly soon -- but again, that would depend on how the Soviets use force, and for how long.Neither would want disarmament talks suspended indefinitely.

Smaller European countries would be less keen to agree to American pressure for anti-Soviet actions.

They have liberal, left-leaning populations whose young people tend to be impatient with anti-Soviet warnings. They have led the fight against the NATO decision to install 572 American Pershing II and cruise missiles in Europe in 1983. Both Holland and Belgium have said final approval for missiles in their countries will be delayed for further review.

The US will hope that a Soviet invasion would at least tip the balance of opinion in the smaller countries toward the need for more arms to match Soviet SS-20 missiles and other expansionism.

Already the edge seems to have been taken off the anti-US missile campaign headed by the Dutch socialists and aimed at left-wing opinion in the so-called "Scandilux" group of countries (Holland, Belgium, Denmark, Norway).

Former West German Chancellor Willy Brandt, has spoken out against the Dutch plans to scuttle the missile program since the US has signaled its desire to push on with arms-control talks.

A NATO special consultative group was planned for Brussels March 31, a NATO nuclear planning group for Bonn April 7 and 8, and a semiannual NATO ministerial meeting in Rome May 4 and 5. By the time the Scandilux countries meet again May 10, pressure on the Dutch may well have escalated.