PARIS -- In the uncertain atmosphere of superpower arms reductions, France and West Germany have swiftly expanded military cooperation, including an increasingly explicit French commitment to defend German territory.

On the ground, the tightening defense relationship between two old enemies has been limited largely to symbolic gestures because of French refusal to participate in the NATO-integrated command that covers West German forces. But in the realm of doctrine, President Francois Mitterrand and Prime Minister Jacques Chirac have gone out of their way recently to emphasize a new French willingness to defend West German soil in case of attack from the East.

A high-level French official explained that much of the eagerness in Paris for military cooperation with West Germany has grown from fears that decreased U.S. nuclear commitment in Europe could lead Germans to look eastward and lower their resolve within the Atlantic Alliance. This has increased as a factor in French strategic thinking since the Dec. 8 U.S.-Soviet treaty abolishing the superpowers' intermediate- and shorter-range nuclear missiles in Europe.

Mitterrand, in a magazine interview Dec. 18, highlighted another major consideration among French officials: the desire to build a stronger "European pillar" of defense within the alliance. Despite France's independent nuclear forces and absence from the NATO command, Mitterrand said, French-German military cooperation should be considered a first step toward such a strengthening and integration of European defenses.

Mitterrand and Chancellor Helmut Kohl of West Germany have laid plans to underline the new level of military relations at ceremonies Jan. 22 marking the 25th anniversary of the Elysee Treaty, French officials said. Long-dormant military provisions of the friendship pact, signed by Charles de Gaulle and Konrad Adenauer in 1963, have been revitalized increasingly by Mitterrand and Kohl since 1983.

The French and German leaders are expected to announce details of a 3,000-man French-German brigade to be stationed in West Germany as a symbol of the two countries' military cooperation. The commander is to be a French general, and the basing site will be near Boeblingen. Kohl proposed the joint brigade last June and his idea was taken up enthusiastically in France.

The brigade, alongside the large Bold Sparrow joint maneuvers in West Germany last September and joint production of a combat helicopter, constitutes the most visible effort so far at French-German military cooperation on the ground. Chirac, in a speech to French officers Dec. 12, said the unit, although largely symbolic, would have been "difficult to imagine as recently as several years ago."

A well-placed French official said a mixed headquarters unit, under the French general starting off a rotating command, should be in place before the end of this year. The entire brigade, ultimately comprising two French battalions, two West German battalions and mixed support and command units, is unlikely to deploy before next year at the earliest.

In a demonstration of the limits to such concrete military cooperation, West German troops assigned to the brigade will have to come from the Territorial Army rather than regular West German forces. This is because regular forces fall under NATO's integrated command, something French forces cannot do under the independent national command set up by de Gaulle when he pulled out of the NATO structure in 1966.

This and similar French-German defense issues are to be discussed regularly in a new joint Defense and Security Council also set for formal announcement at the Jan. 22 ceremonies. The council, which will include ministers, amounts to upgrading a longstanding military council of lower ranking French and German defense experts.

A senior French official said final arrangements for the brigade have been held up by disagreement within the French government, rather than between France and West Germany. Defense Minister Andre Giraud, working under Chirac, has insisted on defining French nuclear protection for French troops in the brigade, he said, while Mitterrand has urged going ahead without formalizing it.

Washington Post correspondent Robert J. McCartney reported from Bonn that such discord in Paris has generated an impression in West Germany that little further progress is possible until the Mitterrand-Chirac rivalry is resolved by French elections.

Although Mitterrand has primary responsibility for French defense policy, the president and prime minister are both likely candidates in the elections, scheduled April 24 and May 8. Chirac, without challenging the presidential defense role, thus has sought to emphasize his own competence in the matter.

The prime minister, for example, has initiated an effort to revive the Western European Union as a forum for defense debates. At his urging, the union issued a Platform on European Security Interests on Oct. 27, in which the seven member nations resolved to speak with a more unified voice on European defense.

In some ways, the rivalry has contributed to hardening France's commitment to defend West Germany along its eastern borders if it is attacked by Warsaw Pact forces. Although France has long pledged to defend West Germany as an Atlantic Alliance member, Mitterrand and Chirac have made it clearer than ever before in the last several months that a French response would be immediate and automatic.

Their statements, which have been closely coordinated, seemed to put to rest the old Gaullist policy that France felt obliged only to defend French territory.

"France would never consider its neighbors' territory a glacis," Chirac said in his speech, adding at another point: "The engagement of France would be immediate and without reserve. There cannot be a battle of Germany and a battle of France."

Chirac spoke after Mitterrand had focused attention on the issue by making a similar pledge during a visit last fall to West Germany. That pledge attracted particular attention because, on the one hand, Chirac heads the neo-Gaullist movement that is heir to the earlier French policy and, on the other, he appeared to make the pledge even more explicit than Mitterrand. Asked about this, Mitterrand noted that Chirac's speech had been cleared by the presidency and declared: "He said the same thing, differently."

France also has moved recently to increase military cooperation with Britain and is considering joint production with London of an air-launched nuclear missile. French officials said Giraud and British Defense Secretary George Younger also have held meetings to discuss coordinated nuclear targeting and coordinated patrols by British and French submarines.