A great many people in Western Europe are now watching closely to see if President Reagan's overwhelming reelection victory will be followed by action, both in Washington and Moscow, towards an improving relationship between the two superpowers.

We see 1985 as a window of opportunity that offers hope of agreements that can check the seemingly endless rise in the cost of defense. But without such hope, West European political leaders will find it increasingly difficult to mobilize public support for adequate levels of defense spending.

Many Europeans fear that if this opportunity is missed, then a quantum jump in U.S. military capability -- with the unrestricted deployment of air-and sea-launched cruise missiles, of the MX missile, of the Trident II submarine-launched missile, of a new "midgetman" intercontinental ballistic missile and of anti- satellite systems -- will lead to a new round of dangerous instabilities in the East-West balance of power.

There is also serious concern in Europe that President Reagan's "strategic defense initiative" threatens the one relatively successful arms control agreement, the 1972 treaty strictly limiting anti-ballistic missile systems.

It is not that Western Europe is weakening in its resolve to uphold democracy and freedom, or that it is unwilling to bear its share of the common burden of defense. There is still overwhelming public support in Europe for NATO as the keystone of western security. There is also widespread understanding of the vital underpinning for that security that is provided by nuclear weapons and the presence of American troops in Europe. There are but a few who do not see the moral position of the U.S. government as of an entirely different and higher order than that of the Soviet Union.

Nevertheless, there is a growing belief that military competition between the two superpowers is as much a cause of East-West tension as it is a result of it. It is becoming clear that adding to western military capability does not necessarily add to western security, when the inevitable response of the Soviet Union is to seek to match new weapon deployments by the West. Isn't there a less confrontational and more constructive attitude to take in our dealings with the Russians?

One result of the American rhetoric of the last four years is that President Reagan is seen in Europe as a man who does not fully understand the impact of his own words, and of the actions of his administration, upon the Soviet Union and upon Europe.

It is, of course, the Russians who need most to be convinced that the United States is not seeking supremacy over the Soviet Union. But many well-meaning West Europeans need to be convinced of that also. Military superiority is neither a sensible nor an achievable goal. It is not sensible because actively to use such superiority would only result in the annihilation of both sides. It is not achievable because the Soviet Union, no less than the United States, will never accept that it should be inferior. Thus, specific American reassurances that the United States does not seek military superiority, and that ''we are ready and willing at all times to discuss and negotiate our differences," as Secretary of State George Shultz said in his recent important speech to the Rand Corp., are as necessary and timely for Western Europe as they are for study by the politburo.

In our judgment of the East-West military balance, we need to interpret "balance" in terms of a concept of sufficiency rather than of equality. Sufficiency involves considerations of doctrine and not just "bean-counting". It recognizes the asymmetries inherent in trying to achieve balance. Sufficiency means giving effect to the statement that we often make that is not necessary for us to match the Soviet Union weapon for weapon, missile for missile or ship for ship. Sufficiency allows full recognition of the view that what is enough to deter the Soviet Union from taking military action against the West is a matter of political and not military judgment.

Surely, in an era of undoubted nuclear overkill we can use our own criteria for setting sufficient levels of our strategic, theater and tactical forces and not have these levels dictated by the actions of the Soviet Union. From this viewpoint, we can use arms control as a process for encouraging stability and mutual restraint, rather than simply as a matter of contractually negotiated treaties. This process would emphasize general trends rather than legally- defined limits and would thus simplify problems of verification.

President Reagan during his first term has restored American self- confidence. Americans again "walk tall." Many Europeans hope that the president will now be prepared to take action, unilaterally if necessary, that would help President Chernenko understand that President Reagan's offer to resume a dialogue with the Soviet Union is more than a tactical move. If he can achieve a return to serious negotiations, this will do much to restore European confidence.

If both sides do not seize this window of opportunity, there is the possibility, indeed the probability, that the technical problems of reaching arms control agreements will outstrip the availability of rational solutions. Time is not on our side. Nor is it on the side of the Soviet Union. In this at least there is a strong common interest between East and West.