THE MASSIVE demonstrations against nuclear weapons, in four European capitals last weekend, followed the outpouring in Germany two weeks ago. They constitute a sharp reminder that American strategic policy has to take more than the Soviet reaction into account. Those demonstrations--in London, Rome, Paris and Brussels--showed how far the European anti-nuclear movement has reached beyond its traditional bases in religious pacifism and far-left politics. The causes of this phenomenon have been ignored by American weapons diplomacy, which has inadvertently exacerbated them.

There's an inherent imbalance in the nuclear relationship between the United States and its European allies. The European governments do not sit at the table at which the United States and the Soviet Union negotiate over strategic weapons, yet the Europeans know that they are as much at hazard as the Americans or the Russians. The Reagan administration has been addressing the Russians in the aggressive idiom of the American conservatives, without much concern for another very attentive audience in Western Europe. It's accurate to say that the substance of U.S. weapons policy has not changed significantly over the past year. But the tone seems, to people on the other side of the Atlantic, to have shifted to a more vehement and threatening pitch.

There's a touch of exasperation in the American response to current European attitudes. The rockets and cruise missiles in question, after all, were offered by the United States as a counterbalance to Soviet nuclear weapons already installed and aimed on Europe. The American weapons were intended only to restore a balance. But the crowds marching over the weekend were moving to a deeper logic. They were protesting what amounts as a practical matter to the drastic loss of sovereignty that the nuclear weapons imply.

Americans need to acknowledge that the decline of sovereignty works both ways in the alliance. True, Europeans aren't formally in the strategic weapons talks with the Russians. But it's also true that the people who were marching over the weekend become an American president's constituents, like it or not, when he gets into these issues. Mr. Reagan has to take them as seriously as if they were demonstrating--and voting--in California or New York. This autumn's peace marchers are conveying an accurate warning of the costs of any defense policy that cannot hold the support of a broad European consensus.