French Defense Minister Charles Hernu warned here today that President Reagan's "Star Wars" missile defense could lead to "complicity" between the United States and Soviet Union that would leave Western Europe to cope with the threat of nuclear weapons on its own.

U.S. development of a successful missile defense, Hernu said, could lead the two superpowers into a "complicity" that "would rid them of any rivalry" in the strategic arena. He said this could leave Europe with a less stable situation than it now has under the strategy of mutual assured destruction to deter a nuclear strike.

Hernu's remarks to the Wehrkunde (defense studies) Conference did not reflect the universal view here, but they did dramatize how far the Reagan administration must go to allay fears in Europe about the space-based missile defense it is pursuing under the name Strategic Defense Initiative. The conference is an annual forum of western military experts and NATO officials.

The current two-day meeting is being held under tight security following the slaying by terrorists in a Munich suburb of a leading West German arms industrialist Feb. 1.

Hernu reflected the views of several European defense specialists here when he said that there is "no assurance" that the Strategic Defense Initiative would lead to a world that was "really stable." He apologized in advance for using the word "complicity" but said that was his "personal opinion" and that he had been urged to speak frankly at the conference. "The risks of instability are real" if a missile defense is developed and deployed, he said.

France is especially worried about U.S. plans for weapons in space because they could render its own nuclear deterrent obsolete. French Foreign Minister Roland Dumas expressed his country's concerns in meetings with U.S. officials in Washington last week.

Chancellor Helmut Kohl of West Germany represented the positive European view of the Strategic Defense Initiative by declaring that the current research phase does not violate the 1972 antiballistic missile treaty and "is a powerful incentive" for the Soviets to negotiate arms reductions.

Declaring that "it is still too early" for his government to make "a final assessment of strategic defense," Kohl said that the U.S. objective is to "maintain and ultimately improve strategic stability."

He said that West Germany and other European allies "must not be technologically decoupled" from the space weapons research program.

Defense Secretary Caspar W. Weinberger said in London yesterday and is expected to repeat at this conference Sunday, that he would welcome participation of allied countries in the missile defense effort.

George Keyworth, White House science adviser, told the conference that the space weapons program "will enhance stability" and "can strengthen the alliance by joint dependence on defense."

Robert W. Komer, a deputy defense secretary in the Carter administration, warned that the pursuit of a leakproof missile defense would take money away from vitally needed conventional forces at a time NATO military budgets are being constrained.

John Gilbert, a member of the British Parliament, said that his country's defense efforts were in such dire straits financially that the military budgets would not grow at all in the coming years.