THE BRITISH Labor Party has now enthusiastically embraced unilateral nuclear disarmament. In West Germany, the Social Democrats seem to have begun sliding closer to the anti-nuclear position, now that they are no longer constrained by the responsibilities of governing. It's true that the cause of nuclear disarmament is represented mainly in parties out of power. But they are responding to public anxiety that is rising audibly across Northern Europe.

In its more defamatory and polemic form, it reduces to the accusation that the reckless and bloody-minded Americans are trying to impose their nuclear weapons on a reluctant Europe. That suggestion is received in this country with justified exasperation. Putting the intermediate-range nuclear missiles into Western Europe was, after all, the Europeans' idea. It was a response to the previous cycle of European anxiety -- which was precisely that the complacent Americans, safe and sound on their remote continent, would never risk nuclear war to defend vulnerable Europe. The European authors of the present embattled strategy wanted the missiles in place in their countries to provide a reliable trigger, ensuring that the Soviets could not move into the West without risking a nuclear response reaching their own homeland.

The West's defense has been based on nuclear weapons from a time before most of today's Europeans were born. Why the sudden surge of opposition? Some of it is clearly a reaction to the careless rhetoric of the Reagan administration, and its strident emphasis on military competition with the Soviets. But the Reagan rhetoric has forced European voters to think about their own governments' decisions, and that's not necessarily a bad thing. A succession of European governments had relied on ultimate nuclear recourse without ever encouraging much public discussion, on grounds that some subjects are best not talked about. As a result, a generation of young European voters is for the first time reflecting on the implications of its countries' policies.

The right posture for Americans is simply to say that Europe can be defended only with those weapons that Europeans choose to support. The choice is Europe's, and Europeans will have to make it. A good many British and German politicians, particularly those on the left, find it more comfortable to deal with this question if they are allowed to say that the only reason for bringing missiles into Western Europe is American pressure. It would be very foolish of Americans to let them off so easily. The real issue is that the costs of an adequate conventional defense of Europe, in terms of men and money, are quite a lot higher. That debate is well left to Europeans. The tone will frequently be anti-American. But the substance of this quarrel is the Europeans' attempt to find, in this generation, a solid political consensus on which to base their own defense.