A European who remembers the heated and bitter debate over whether U.S. Pershing and cruise missiles should be deployed must ask for understanding if he finds it difficult to follow the present American advocacy for strategic-missile defense.

In the European debate, anti-nuclear protesters, the political left and many men of the church were all claiming that deterrence was no longer acceptable as the basis for Western security. It was, they said, profoundly amoral to threaten the destruction of another society. As we know, their protest did not succeed. But now no less a person than the president of the United States is saying virtually the same:

"The human spirit must be capable of rising above" -- March 1983.

"Would it not be far more humanitarian to say that now we can defend against a nuclear war by destroying missiles instead of slaughtering millions of people?" -- October 1984.

"There is a better way of eliminating the threat of nuclear war than retaliation with a deadly counterstrike" -- February 1985.

What makes these remarks so disturbing is not that they may or may not be correct sometime in the 21st century but the effect they have today, in the 1980s and 1990s. At this stage, nobody knows if there ever can be, from a technical point of view, a reliable defense against ballistic missiles; most scientists doubt it. Nobody knows all the countermeasures that a determined enemy might develop. And nobody knows whether such a system could ever be funded -- the price tags currently quoted of around $500 billion are as speculative as the rest of the exercise.

On this shaky basis, caution rather than enthusiastic advocacy would seem the order of the day. One would expect a serious research effort to identify what is possible in defensive systems, coupled with persistent emphasis that, for the foreseeable future, there is no alternative to the "balance of terror" as the underpinning of our security. Yet we are witnessing something very different. From the highest authority of the Western World comes a string of statements that the dreadful, morally repugnant days of deterrence through the threat of retaliation are numbered and the bright, new and morally sound alternative of strategic defense is nigh.

If deterrence were a robust doctrine with strong public support, one could dismiss these statements as the typical American way of presenting visions as if they were reality. But deterrence is far from enjoying popular support. It has taken a serious knocking in recent years. Skepticism, doubts and outright rejection have grown in all Western societies. There are many reasons for this. The spectacle of an unrestrained arms race has raised fears where all this might end. The deadlock in East- West arms control has disappointed many hopes. Military men and civilian strategists have pretended that somehow the waging of nuclear war could become a serious military option. The peace movements have frightened themselves and others that deterrence is driving the world to certain disaster.

The net result is that deterrence through the threat of nuclear retaliation has become a brittle concept -- not vis- our adversaries but in the eyes of our own citizens. And yet, to date, it remains the only concept we have for survival in the nuclear age. It has helped to keep the peace between East and West for 40 years and, unless political leaders should lose all caution, is likely to continue to do so. To fight a nuclear war may well be immoral. But to threaten nuclear retaliation in order to prevent an attack is a very different matter.

Moreover, we may have no choice. If past experience is any guide, the attempt to escape from the nuclear dilemma through strategic defense will end up like all other attempts in history that sought to replace offense by defense: with a new arms race and, despite major efforts, no basic change.

If the Strategic Defense Initiative should escape this fate, it would be little short of miraculous, something to believe when it happens but not to bank on now. The difference between those in favor and those against SDI is whether, in some distant future, defenses against ballistic missiles could effectively constrain a determined enemy attack. That remains to be seen. In the meantime, as Paul Nitze, special adviser on arms control to the president and the secretary of state, has noted, for "at least the next 10 years, we will continue to base deterrence on the ultimate threat of nuclear retaliation. We have little choice; today's technology provides no alternative."

However, the constant attempts from the White House to talk strategic defenses up by talking retaliation down will make it increasingly difficult to regenerate and maintain public support for the West's nuclear doctrine. And what if, at the end of all the research, a strategic defense, the bright new world, fails to materialize and we arrive where we started -- in the old, grey world of deterrence?

The enthusiasm for SDI is thus an example of the familiar human failing of constructing the future before mastering the presesults allow for a proper examination, it is an example of strategic escapism. There is, for a long time to come, no alternative to keeping the peace by threatening an enemy with nuclear destruction. Political leaders, instead of straining further an already brittle consensus in our societies, should seek to strengthen it through realism and not weaken it through illusions. In the nuclear age, you play around with deterrence at your peril.