President Reagan has presumably purchased some increased credibility for his administration's arms control intentions with his modification of the United States plan for dealing with the Euromissile problem. The proposal implicitly acknowledges the impracticality of the administration's original zero-option objective, which would have required that the Soviet Union dismantle all of its SS-20 missiles (now totaling 351, each with three warheads) in exchange for no US deployments of ground-launched cruise and Pershing II ballistic missiles in Western Europe.
Does the shift in the US position mean that we are any closer to an agreement in Geneva? Perhaps so. But the President's new proposal enshrines a concept, also implicit in the zero-option proposal, that could foreclose prospects for an arms control agreement.
The potential flaw in this approach, at least in terms of arms control prospects, is the insistence that the negotiations result in numerical equality in US and Soviet warheads on intermediate-range nuclear missiles. Numerical equality, on the surface, sounds a reasonable principle. But when applied to a narrow range of weapons systems, numerical equality may be a standard that is both irrelevant to Western security requirements and fatal to arms control prospects.
Nuclear weapons serve different purposes in NATO nuclear strategy than they do in Warsaw Pact strategy. Given different strategic assumptions about the purposes such weapons would serve and what missions they would be assigned, neither NATO nor the Warsaw Pact has in the past attempted to develop systems that mirror those of the opposition.
In Soviet strategy, the systems such as the SS-20, and the SS-4s and SS-5s that predate the SS-20s, probably are intended to ensure that no future war is fought on Soviet territory, to help deter Western use of nuclear weapons against the Warsaw Pact, and to ensure the best possible ratio of Eastern and Western forces in peacetime as well as to provide coverage of key West European targets in a war. About 100 of these missiles, deployed in the Asian portion of the Soviet Union, clearly are designed primarily as a deterrent against China.
Even if we acknowledge these objectives as legitimate from a Soviet point of view, deployments of SS-20s now would appear to have outstripped their legitimate requirements. The West therefore has a strong case to make on behalf of Eastern reductions.
NATO nuclear planning, on the other hand, has other purposes. The new US systems were originally intended in no way as direct military counters to the SS-20. Rather, they were designed primarily to serve NATO deterrent strategy by strengthening the linkage to the US strategic nuclear guarantee while providing Western authorities with a more flexible range of options with which to deal with any Warsaw Pact attack. The new systems are a response to the SS-20 primarily in a symbolic political sense.
Furthermore, there is such an intimate relationship between the new US and Soviet systems and other medium-range and strategic nuclear weapons systems that separating out one component of the balance from the others is an artificial exercise at best.
Under these circumstances, Western security depends on an overall balance between Western and Soviet nuclear capabilities. It may also depend on reductions in the overwhelming numbers of SS-20s the Soviet Union has deployed and on Western deployment of some new missiles to ensure a continuous spectrum of deterrence against a Soviet threat. Deterrence does not, however, require equal numbers of these systems.
Reagan's insistence on numerical equality between warheads on US missiles in Western Europe and Soviet intermediate missiles in Europe and in Asia is a significant shift from previous US and NATO positions.
Item: NATO's original 1979 decision to deploy 572 new missiles in Europe was based on a number of criteria; those criteria did not include an attempt to produce parity in such systems with the Soviet Union.
Item: NATO's 1979 arms control offer sought equality of ''rights and ceilings'' in an eventual agreement, but not equality in numbers.
Item: The informal proposal worked out by the US and Soviet negotiators last year - the ''walk in the woods'' proposal - reportedly would have produced equality between US and Soviet intermediate-range missile launchers in Europe, but would have allowed the Soviet Union to keep a certain number of additional missiles in the Asian portions of the Soviet Union.
The history of this issue suggests how substantially President Reagan has diverged from previous policies. Unless the Soviet Union is smitten by a sudden streak of generosity, no agreement will be possible unless the US relents from this requirement.
Because the requirement for numerical equality now is more firmly entrenched in the US position, it may be that the only way in which intermediate-range missiles in Europe can ever be limited would be in the context of an overall accord on reductions in strategic nuclear weapons systems. Any such accord would have to allow the two sides a certain degree of flexibility to choose among systems that would constitute the overall ceilings.
Within such a framework, it is possible to conceive of eventual reductions in the numbers of European-based US and Soviet nuclear weapons systems. Without such a link, the prospects for controlling such systems in Europe remain problematic at best.