The United States is reducing the number of its nuclear weapons in Europe, despite plans to station new medium-range nuclear missiles here, Lawrence C. Eagleburger, assistant secretary of state for European affairs, said today.

Pressing the Reagan administration's counteroffensive against the so-called peace movement here, Eagleburger warned that the "mixture of fear and wishful thinking," which he said is driving European opposition to the 1979 alliance decision to deploy new U.S.-made Pershing II and cruise missiles in Western Europe, threatens not only Europe's survival but America's as well.

"I can understand concern with the numbers of nuclear weapons on this continent," Eagleburger told a gathering of parliamentary deputies from NATO's 15 countries during a meeting in Munich of the North Atlantic Assembly. "But I never hear reference to the fact that last year the United States withdrew 1,000 nuclear weapons from Europe, without replacement."

While the reduction, from 7,000 to 6,000 warheads, was part of the original alliance decision, U.S. officials had appeared reluctant to publicize it for fear of sending the Soviets the wrong signal.

Highlighting this fact now suggests that the Reagan administration has realized full well that it must campaign for public support in Western Europe. The Soviets, for their part, have been claiming that the deployment of their new triple-headed SS20 nuclear missiles -- which the planned American deployment is intended to offset -- also involves a reduction in launchers.

He ended his lengthy speech with a warning against the risk of being blind to aggression, as some once were to Adolf Hitler. "Forty three years ago this month," he said, "Neville Chamberlain traveled to this city Munich in search of 'peace for our time, peace with honor.' Within months, Europe was once again at war. A combination of fear, wishful thinking and misguided idealism led to disaster. It must never happen again."