BRUSSELS -- This week's NATO summit meeting should have been the birthplace of a new western strategy for responding to the diplomatic offensives of Soviet leader Mikhail Gorbachev. But the separate agendas that the leaders of NATO'S Big Four nations have brought to the Belgian capital ensure that the two-day gathering will fall far short of that lofty goal.

Grand politics, rather than grand strategy, is the order of the day. The final communiques will have already been agreed upon and reduced to vague generalities before Wednesday's opening session. This minimizes the risk that an intensive debate on a more coordinated alliance strategy for dealing with Gorbachev will erupt here.

As seen from the White House, the summit will serve as a valedictory celebration of Ronald Reagan's leadership in U.S.-European affairs. The gathering may also give an added boost for the president in the Senate debate over ratification of the Intermediate-range Nuclear Forces (INF) arms control treaty he signed with Gorbachev last December.

Reagan is likely then to leave Brussels Thursday without the Europeans bringing up the serious problems they feel that both this president's leadership and his treaty have created for the alliance. Not even Britain's combative prime minister, Margaret Thatcher, is prepared to rain on the president's NATO sunset.

While generally favoring his first-term buildup of the U.S. nuclear arsenal, Europeans were unsettled by the strong streak of unilateralism that has run through Reagan's "Star Wars" plan, his willingness to bargain away nuclear deterrence at Reykjavik and the manner in which the INF treaty was negotiated over their heads.

This pattern of American self-absorption has spurred the most serious discussion of European defense cooperation since the 1950s, with France and West Germany drawing plans for experimental joint military units and France and Britain gingerly beginning to discuss nuclear strategy. There may be a statue to Reagan here some day as the father of a modern European defense strategy.

But instead of using the Brussels summit to address the problems of European defense directly with the Americans, European leaders have other fish that will be easier to fry if Reagan moves off the NATO stage with a hurrah rather than a set of detailed ideas about flexible response.

Thatcher, who is now the most experienced and domestically secure leader in Western Europe, is well positioned to use this summit to become the "caretaker" of alliance interests in this American election year, and perhaps beyond if Reagan's successor appears to her to be a hopeless wimp. She expects to host a 40th anniversary NATO summit -- and to introduce the new American president to Europe from that stage -- in London next year.

She staked out her claim to lead the search for a new defense and detente consensus two weeks ago in hawkish remarks blasting Gorbachev in a closed-door session with NATO ambassadors here, while her foreign minister was in Moscow extolling the new warmth of Soviet-British relations. By promptly publicizing her remarks in Brussels in the British press, Thatcher's aides seemed to confirm an interest in a larger NATO role.

They had earlier passed the word that Thatcher will authorize the stationing of up to 60 more F111s in Britain to take up the nuclear slack she fears is being left by the INF treaty. She would make Britain the new European nerve center of flexible response.

That divides her from France, where President Francois Mitterrand has responded by reopening the old French quarrel with flexible response. Some French officials also attribute Thatcher's verbal thrashing of Mitterrand and Prime Minister Jacques Chirac last month in London about French-German defense cooperation to her eagerness to keep alliance affairs uncluttered at a time of her ascendancy.

The timing of the summit is opportune for Mitterrand, who is expected to announce in a week or two his candidacy for reelection, and for Chirac, who is already in the race for president. They both put aside France's traditional reserve about such gatherings by agreeing to participate fully in the Brussels photo opportunity.

West German Chancellor Helmut Kohl's contribution was to get Reagan to agree not to press Bonn at the summit on accepting a new generation of battlefield nuclear missiles. The communiques that advance teams have drawn up also muffle the argument over opening negotations with the Soviets on such weapons.

In the end, the planning for the Brussels summit became an exercise in seeing, hearing and speaking no conflict. It made for a better photo opportunity, but not for better policy.