West Germany and France today opened formal discussions on military and strategic cooperation that will have important implications for Western defense.

The six-hour closed session, involving Foreign Ministers Hans-Dietrich Genscher of West Germany and Claude Cheysson of France as well as Defense Ministers Manfred Woerner of West Germany and Charles Hernu of France, coincided with the arrival of French President Francois Mitterrand for a two-day visit. It was the first trip to Bonn by a European head of state since Chancellor Helmut Kohl's election by parliament Oct. 1.

While the West German and French leaders were expected to touch on a variety of economic, East-West and other policy questions, the new emphasis on consultations in the security field marked a major development in relations between the two neighboring continental powers since their postwar reconciliation two decades ago.

A Bonn government official said beforehand that this initial meeting on strategic affairs would likely deal primarily with establishing procedure and topics for future sessions, which are expected to come on a regular basis.

West Germany has long been interested in learning about the defense plans of France, which withdrew from the military wing of the North Atlantic Treaty Organization in 1966 and has been highly reluctant to share its strategic blueprints with other Western powers since. Some French forces stationed near the West German border are equipped with nuclear-tipped Pluton missiles, which have a range of 72 miles, meaning they could well be exploded on West German soil in the event of a Soviet attack.

But the initiative for the new talks reportedly came from Mitterrand, who broached the idea in a meeting with then-chancellor Helmut Schmidt in Paris last February.

On one level, what originally motivated the French was the prospect of West German financial support for joint armament projects favored by Paris. The Mitterrand government has made clear its intention to give priority to a buildup of nuclear forces at the expense of new conventional arms development. But some conventional firepower, it was thought, could be enhanced with West German assistance. Joint development of a tank and, more recently, of an antitank helicopter, are on France's wish list.

In addition, several French newspapers in the past week have carried unconfirmed reports of plans to cut back the 50,000 French troops who are part of three armored divisions stationed in West Germany. Any change in the French military presence here would be a very sensitive decision politically for both France and West Germany.

As a result of Bonn's own worsened financial condition, there are indications that French expectations of a West German bailout may have waned. But on other political and military strategic levels, cooperation with Bonn makes sense for France.

Politically, the French, who have been exceptionally worried about a drift toward neutralism here, reportedly see in the new talks an opportunity to bind the West Germans more closely to Western Europe by offering some insight on French defense planning.

Strategically, too, France may be moved by its anticipated development of new tactical nuclear weapons to consult the Germans about a possible accompanying refinement in its military doctrine away from a strategy of massive nuclear retaliation and closer to the Western alliance strategy of flexible response.