IT DIMINISHES the talks that the United States and the Soviet Union are opening in Geneva today to say that they are about limiting a certain class of weapons in Europe and preserving a military and psychological balance there. The talks are about something more important: limiting the risk of nuclear war. The European peace movement, however skewed its judgment on some of the particulars, understands this fundamental point well. Not so many Americans feel the rising threat of war that many Europeans do. But in its various forms the fear is there. There are too many bombs located in and trained on Europe. The balance is too fragile. It is an imprudent person who is not alarmed and determined to make Europe a safer place. Without a continuing focus on this larger obligation and purpose, the talks in Geneva cannot possibly succeed.

Ronald Reagan did not arrive at the White House proclaiming a sense of the danger of nuclear war. On the contrary, he came insisting that American policy had been gratuitously enfeebled by a flight from the harsh reality of Soviet expansionism. He and his aides did not preach war. They did, however, preach a risk-taking philosophy that they contended would make war less likely but which many others perceived in precisely the opposite way. What Mr. Reagan saw as a necessary assertion of will and national purpose was to many listeners frightening and reckless. This set up a series of rumbles of public opinion, especially in Europe, that gave a wide opening to Soviet propaganda. Only by his announcement of a new policy on arms control negotiations was Mr. Reagan able to check this current of feeling.

It will take some time to see just how the administration means to add arms control to the arms building that is already established as its principal manner of confronting Soviet power. He hopes to open soon negotiations on strategic arms, conventional force levels in Europe and surprise attack in Europe. The talks opening today, Mr. Reagan says, are on medium-range missiles based in Europe; the Kremlin says they are on all medium-range systems. These systems include, on the Western side, American aircraft and the French and British nuclear bombs. Whether they should be counted in may turn out to be the central dispute in Geneva.

The administration says no and, as we understand it, its reasoning runs like this: the old measure of equality, based essentially on the number of missile launchers, needs to be replaced by a measure based on equality of deterrent effect. That sort of equality existed from the 1960s into the late 1970s. But then Moscow began deploying new, very accurate and very mobile--and therefore very menacing --SS20s. The American aircraft in Europe are no real match because they cannot be relied on to get through the formidable Soviet anti-aircraft defenses. The British and French deterrents are also no real match because of the separate and substantial political inhibitions on their use. Only the new missiles the U.S. plans to deploy in Europe, especially the very accurate and mobile Pershing IIs, would be a match. To avert that threat, or so the Reagan theory goes, the Soviets should be willing to forgo their SS20s.

It is a serious theory. But even a serious arms control theory can only succeed in the context of an overall East-West policy that is plausible and coherent-- and of which the arms control initiatives are seen to be an essential, consistent part. By going to Geneva, Mr. Reagan puts himself and the credibility of his policies to a new and necessary and demanding test.