Although Ronald Reagan's speech setting forth his vision of a Star Wars defense was made nearly three years ago, it is still causing sleepless nights for the top brass of France's first-rate military-industrial complex and headaches among its political elite.

This is a paradox for a country that has built itself up into 1)the world's third nuclear power, with a fast-growing arsenal, and 2)the No. 3 power in space, with leadership in Europe.

One might have thought that France, with such a level of technological excellence, would be all the more eager to discuss the Strategic Defense Initiative and to contemplate acceptance of the American offer to participate without psychological inhibitions. Moreover, France has been traditionally less affected by the brain drain phenomenon than its European partners.

Yet it is Britain which has signed an agreement with the United States on SDI and Germany which is getting ready to do so. Italy has been interested from the very beginning.

The French government frowns at these deals, arguing that America is not serious about sharing its technology and will ultimately give its allies nothing but SDI "crumbs."

France might be wrong. But for once it is not simply trying to play odd man out or to be a nuisance for the fun of it. Confronted with SDI, France has got a real dilemma.

Given the huge disproportion between the size and composition of the French and Soviet arsenals, France's nuclear doctrine remains the former American credo: deterrence through massive retaliation. French missiles are designed not for counterforce strikes and war fighting, but for massive blows against Soviet cities and industrial centers.

Even France's "tactical" nuclear forces are not tactical at all. According to the doctrine, their purpose is not to destroy targets in a battle, but rather to give the enemy a "last warning" before launching the all-out strategic nuclear attack.

Thus, France is the last true believer in mutual assured destruction, and as such officially opposes SDI. The fear is that the American program will prompt the Soviet Union to multiply its ABM capacities and -- to use President Reagan's own words in his Star Wars address -- "make nuclear arms impotent and obsolete" -- but on the French side.

Engineers in the French Atomic Energy Commission and in the Defense Ministry either do not share this fear or more likely suppress it, maintaining that they have the technology to deliver the bombs on target, even if the Soviets deploy a heavy net of ABM defenses.

Nevertheless, SDI cannot be wished away. As Alain Minc, one of the most provocative of France's young political intellectuals, remarks: "The mere discussion of these ideas creates a new strategic reality."

As an alert and astute statesman who immediately grasped the political dimension of the Euromissile challenge when he came to power in 1981, Francois Mitterrand quickly realized that he could not shirk the SDI issue. This time, however, it was not easy to draw a clear path between conflicting priorities. Hence, a string of ambiguous or contradictory pronouncements.

First, (at The Hague in February 1984) Mitterrand invited Europe to gear up for a military presence of its own in space. But the idea was never picked up again until a recent press conference. In between, he had the French representative at the disarmament conference in Geneva condemn SDI (June 1984) -- a move that the Soviets have tried to make the most of ever since.

Then suddenly last April, with much ado but a certain lack of preparation, he launched Eureka, a "grand design" in the Gaullist style. Through a major combined effort in civilian high-tech research, Europe was "to take on the technological challenge" presented by SDI.

Now, nearly a year later, despite some contracts and continuing discussions, it appears that Eureka will not live up to the promise of its name. Throughout, and without much publicity, French firms had been authorized to explore possible SDI contracts as they saw fit.

These mixed signals have generated a feeling of confusion. The leaders of the opposition are beginning to capitalize on this "incoherence." Two of the main contenders for the premiership after the legislative elections next month, former president Valery Giscard d'Estaing and former premier Jacques Chirac, have expressed their interest in closer and more straightforward cooperation on SDI -- while remaining within the framework of the French nuclear doctrine.

In the end, France is having trouble reconciling the seing contradictions between its strategic doctrine and its ambitions in space. Whereas Japan and West Germany could hope that a deal on SDI would allow them to bypass the nuclear club -- in which they had been denied membership -- France fears that if it stresses the importance or feasibility of space weapons, it will endanger the very basis of its own nuclear status.

Yet, as a successful newcomer in space, it sees clearly -- even more clearly than its neighbors -- the shape of things to come. And so France is pushing for two major projects: in its view the Ariane 5 rocket and the mini- shuttle Hermes are the building blocks for the European civilian presence and, perhaps one day, a European military role in space.