On the right weekend, with the right train connection, you could have been in one demonstration of the European "peace movement" here, in the home town of the North Atlantic Treaty Organization, and a significantly different demonstration of the same phenomenon in Paris--on the same Sunday. Or you could have had your pick of "peace" marches on Saturday in London or Rome.

When you add this one weekend's outpourings to the kickoff rally in Bonn more than two weeks ago, the conclusion seems inescapable: just as the anti-war movement of the late 1960s contributed significantly to the collapse of Lyndon Johnson's cherished Vietnam consensus, so a wave of protest in Europe could engulf the critical element in the projected buildup of NATO's nuclear defenses.

That element is the "double track" approach, agreed to in December 1979 and subsequently reaffirmed, by which new American Pershing II and cruise missiles would be deployed in Europe two or three years from now. That's one track. The second track is a negotiation effort with the Soviet Union starting next month to reduce or eliminate the comparable Soviet SS20 "theater nuclear forces" (TNF) already ominously in place.

Either way, by deployment or negotiation, the United States is determined to redress what is perceived to be a grave threat to effective nuclear deterrence of the Soviets. It is the Soviets' plain aim to undermine a patchy and shaky European acceptance of the "double track."

So it is tempting to read into the European "peace movement," first of all, a communist conspiracy--coached, encouraged and controlled from Moscow, manipulated covertly by the KGB. And that's a part of it, even though American officials freely admit they are unable --not for want of trying--to pin down conclusively a direct or decisive Soviet role.

"There has to be some money, at least, from the East," says a high-ranking American at NATO headquarters, "but we can't find the fingerprints." Small wonder, when you consider how the fingerprints are smudged. Nearly 1,000 organizations--ecologists, the Protestant church, women's liberation groups, trade unions, traditional pacifists--are said to have sent delegations to the Bonn march. An estimated 30,000 Dutch were bused in.

In Paris, the French Communist Party and the communist trade unions were conspicuous in their management of the affair. Anti-Americanism was a dominant theme. Mothers rolled their children by in baby carriages. Their signs read: "Mr. Reagan, please let us grow up."

But the focus was not all that sharp. One brave orator denounced Soviet SS20s with equal fervor--and was roundly booed. Here in Brussels, placards denouncing Soviet TNFs figured prominently in the march. And while the local Communist Party's hand showed, so did that of the church. "Many tributaries feed the campaign for nuclear disarmament," said a long and thoughtful editorial in The Times of London on the day of the demonstrations there. Leave aside the communist content and the analogy with Vietnam still holds. The American peace movement drew disparate support as well. But the analogy breaks down in interesting--some would say promising-- ways when you get around to examining European hopes and fears.

Ask a European politician or government official what's really going on here and the composite response goes something like this: it is more than simple pacifism, or unilateralism, or anti-Americanism, or neutralism, or nationalism or, as one put it, "indifferentism." It is too multi-generational to be dismissed as a matter of only radical or turned-off youth. It is the sum of a little, or in some cases a lot, of all of these things.

To which some would add a dark, unanswerable fatalism--a sort of sandwich board statement by a significant slice of Europeans that the end is near. Europeans too young to know war or old enough to know its ravages too well come out in the same place: both are baffled by the subtlety and appalled by the language of deterrence and are against nuclear weapons of any kind.

What's promising about that? Only the possibility that some part of this composite can be dealt with, to some degree. One way is forceful and forthright promotion of measures, however dim their prospect, already in sight: strategic arms negotiations promised for early next year, and that second TNF negotiating track.

But an equally important effort needs to be made to explain (in less hair-raising terms than Ronald Reagan has recently used) what the concept of deterrence is all about.