BRUSSELS - As the deadline for President Bill Clinton's decision to go ahead with a national missile defense approaches, the need for clear thinking becomes urgent.

NATO's European members do not dispute the United States' right to enhance its national security, but they question unilateral U.S. action in this instance. At a recent North Atlantic Council meeting, their foreign minsters voiced concerns regarding the nuclear balance, arms control, disarmament, international stability and a gradual decoupling of the United States from Europe.

The idea is to provide absolute protection for American territory against an attack by a small number of intercontinental missiles with rudimentary penetration aids. The system would not provide defense against Russia's nuclear arsenal.

Nor would it attempt to safeguard against potential terrorist attacks with chemical or biological agents or small nuclear devices capable of inflicting similar damage. Its sole purpose would be to inhibit a possible missile threat from an irrational opponent to destroy a major American city in order to gain a political advantage.

Deployment of 20 interceptors at a single site in Alaska by 2005, with the associated identification and guidance equipment, would not be a major upset of the nuclear balance. And the cost would be a reasonable proportion of the U.S. defense budget.

Although two of the three pre-decision tests have been failures, deployment within the planned timeframe is certainly not beyond American technological capabilities.

However, such a shield cannot provide the required absolute protection for any length of time, as technology inevitably spreads across the globe. Thus the program foresees an increase to 100 interceptors by 2007 and, in anticipation of a technically more sophisticated threat, to 250 interceptors with a much greater quantity of associated equipment by 2011. Costs would escalate in proportion.

At present the United States is prohibited from deploying such a system by its Anti-Ballistic Missile Treaty with Russia. The treaty binds each party not to deploy an ABM system in the defense of its territory and limits such defenses to a single site containing no more than 100 missiles, as well as restricting the associated radar equipment. The treaty provisions may be amended, and either party can withdraw from it with six months' notice.

The Clinton administration's plan is to negotiate changes as required. President Vladimir Putin refuses to consider this, claiming that any change would upset the present balance.

This balance rests on the nuclear deterrence principle of mutual assured destruction, whereby during the Cold War each side was able to destroy the other even after receiving a first strike. The limits imposed by the ABM Treaty are a crucial element of this principle.

Powerful American political elements no longer believe in the deterrence concept; they consider the treaty outdated and want abrogation. Instead they advocate not only the present missile defense program but its extension to produce an impenetrable shield for the whole of the United States against all types of ballistic missiles. This would have several unpleasant consequences.

In Asia, the Chinese reaction has already been understandably strong; even the present missile defense plan would make China's missile capability ineffective and probably spur efforts to improve and enlarge it. This would then call fortha similar reaction in India, which in turn would activate Pakistan.

In Europe, the working relationship with Russia, whichis an important NATO objective, would inevitably go into decline. President Putin has already warned that any amendment, let alone abrogation, of the ABM Treaty would ruleout Russian implementationof the recently ratified START-2 treaty. Negotiations for START-3, which are already well advanced, would stall.

Inexpensive methods of expanding Russia's nuclear arsenal, such as multiplying the number of warheads on existing missiles, could fairly quicklybe applied.

The U.S. project would therefore trigger another arms race, however irrational this may seem in the present major-power relationship.

Security experts have to look at the capabilities of a potential opponent and advise action to guard against them. But such capabilities do not arise in isolation; they result from political aims and their dynamics. A defensive shield that has offensive side effects can be only a temporary solution, and since other types of threat exist it can offer only partial protection.

It is surely more urgent to profit from the improving international climate to shift the emphasis onto dialogue, with political and economic incentives, resulting in treaties reducing and eliminating nuclear weapons and their carriers. American technology is capable of providing the required stringent verification processes as well as the punitive power to ensure that such agreements are not breached.

The writer is editorial director of NATO's Nations, an independent military journal. He contributed this comment to the International Herald Tribune.