McNamara was among the few strategic thinkers of the time who had grasped the fact that such a defensive shield was in fact an offensive weapon, behind which one nuclear power could shelter while launching its missiles in a "first strike" at another. Therefore, building such a shield inevitably would tempt that other nuclear power to build up its missile force, extending the arms race, in order to overcome the shield.

When Kosygin and his successors finally accepted this argument in about 1969, the ultimate result was the Anti-Ballistic Missile treaty. The Soviets entered into that agreement in 1971, with the Nixon Administration. Decades of nuclear peace, underwritten by the treaty, have continued to this day.

It's a mystery, therefore, why President Bush has chosen this particular moment or any moment -- to pull the United States out of the ABM treaty, over Soviet, Chinese and allied objections. That he keeps calling the treaty merely a "relic of the Cold War" is no explanation at all.

Some relic, anyway! To which civilization itself, including George Bush’s generation of Americans, may well owe their survival. Besides, even if the treaty were nothing but a relic, that would be no good reason to abandon it. Relics deserve honor and preservation, even if in museums or archives -- not the back of an impatient hand. Even if repudiation were a good idea, moreover, the moment is hardly auspicious. While the attacks of Sept. 11 did demonstrate a terrorist threat to the United States, they would not have been prevented by even the most elaborate missile defense. In fact, if those attacks proved anything, they proved how vulnerable the U. S. is to low-tech terrorist attack. A suitcase bomb, say, or a truck bomb like the one that destroyed the federal building in Oklahoma City.

The United States -- Secretary of State Colin Powell, at least -- is trying to hold together a tenuous world coalition against terrorism. A single-handed U. S. decision to deploy a missile defense, with all its ramifications but little regard for allied opinion, can hardly be helpful. Indeed, the Bush Administration already has aroused fears that it tends bullishly to go its own way in the world.

Some of the President’s remarks early in his Administration encouraged that belief. Just recently, U. S. representatives proposed delaying talks to strengthen the 1972 Biological Weapons Convention, which they said would damage U. S. business interests. The Senate has just enacted a provision to keep U. S. military men out of the jurisdiction of an international criminal court. Now comes unilateral abandonment of the ABM treaty. No wonder many a foreign diplomat is wondering "what next?"

Perhaps more damaging than any of this is the polite but definite slap in the face to President Putin of Russia. He has gone out of his way to support the U. S. war on terrorism and to strengthen Moscow’s relations with Washington. He has repeatedly made it clear that, under the existing ABM treaty, he cannot accept the Pentagon's proposed anti-missile tests; and he has told Rumsfeld directly that these tests would "gut" a treaty that he regards as a milestone, not a relic. That the U. S. nevertheless intends to scrap it, over his objection, may well damage his domestic political standing, hence his efforts to improve relations with Washington.

Beyond these more immediate problems with abandoning the treaty ? The first renunciation of a major arms control agreement in the nuclear era, and by the U. S. ? the original and more fundamental objections remain: Deploying a missile defense will revive McNamara's old argument that such a defense is really an offensive weapon; China in particular already has threatened to build up its own nuclear missile force if the U. S. goes ahead.

That suggests the possibility of a new arms race ? particularly since even Russia cannot be indifferent to a Chinese missile buildup. India, moreover, China’s ancient enemy and a fledgling nuclear power, will almost certainly respond to China’s expansion of its missile capacity. Whereupon, Pakistan, India’s historic antagonist, might be forced to do the same.

Aside from the obvious dangers in such an arms race, these Asian countries have a desperate need to devote their resources to the improvement of their people’s lives, The money they may spend on missiles, resulting from the Bush-Rumsfeld decision to deploy a missile defense around the U. S., will be money that should have gone into health, education and economic development.

Nor can the U. S. regard the cost of missile defense with indifference. Even the ground-based system tentatively advanced by the Clinton Administration, if carried to deployment, was estimated to cost $60 billion. The Bush Administration’s much broader plan, for air- and sea-based defenses as well, will cost billions more ? not counting the inevitable cost over-runs, false starts and failed tests and systems. These are certain, because the most fundamental objection to missile defense is that it always has been, and may well be still, technologically unworkable. The billions poured down the rat hole of President Reagan's visionary but failed "Star Wars" scheme may be only a down payment on the further billions to be expended in pursuit of the Bush-Rumsfeld decision to plunge boldly into the unknown.

Perhaps into the unnecessary, too. For if the immense obstacles that always have prevented the success of missile defense should be overcome, and if the necessary truckloads of dollars can be found, an ultimately deployed missile defense might form only a Maginot Line in the sky, easily by-passed by suitcase or truck explosives and suicide bombers, right here on earth.