AT LEAST AS Secretary of State Alexander Haig reads it, NATO policy in the event of a Soviet conventional attack on Western Europe might go something like this: Soviet tanks begin to roll across the continent, overrunning weaker NATO conventional forces. The allies, unsure how else to react, turn quickly to the nuclear arsenal. From it they select a single bomb and explode it somewhere in the sky over Europe, demonstrating our readiness to fight a nuclear war.

It may be, as an embarrassed Defense Secretary Caspar W. Weinberger '38 insists, that NATO plans no longer call for a demonstration explosion. But even if it's only a fantasy of our nation's chief foreign policy administrator (and one should notice that the White House did not disavow his statement), that is scary enough. For Haig's scenario demonstrates the ultimate idiocy of any nuclear deterrent: what happens when your bluff is finally called.

Other incidents also point up the precariousness of our nuclear peace--off Sweden, a Soviet submarine probably loaded with nuclear missiles blatantly violated the territorial limits of a nation. In Washington, our president says he thinks the Soviets think they can win a nuclear war. And administration officials talk freely about the possibility of fighting a "tactical" nuclear war that would not grow into an all-out conflagration.

Scientists give us 500,000,000 years or more before the sun grows big enough to sautee mankind; that is, they add, unless we do the job first. An all-out nuclear war, most rational observers contend, might well make the species extinct and would certainly end modern civilization. Now in that 500,000,000 years--longer than man has already been on this planet--many thousand ideologies will rise; many nations and races win power and lose it; and, one hopes, much progress toward true justice be made. Our goal, then, must be justice and freedom in our lifetime, but only if it is possible without courting the nuclear disaster which would end all chance for future progress.

Haig's plan--and any like it--are the recipes for that nuclear disaster. Our defense is currently based on a credible nuclear deterrent. The arsenal needed to make good that threat implies, and certainly allows, a nuclear response if the deterrent fails. The "demonstration" explosion scenario merely carries the threat closer to the actual; in case anyone has forgotten Hiroshima and Nagasaki, the reasoning goes, this will prove our willingness to use such weapons. And what if the tanks keep rolling? Certainly the chances are good that such weapons will be used and Armageddon will be triggered. The trouble with basing a defense on an ultimate deterrent is that only one failure of deterrence will leave the world in ashes. Given the long history of human folly, it would be silly to expect such restraint to last for 500,000,000 years, or even 500.

Our aim, then, must be to take away the source of danger, not to figure out ways to buck the odds against oblivion for a few more years. Thankfully, there seems to be a growing recognition of this most important of facts. In Western Europe--and even in Rumania--people seem to be saying that no, they'd rather not have a demonstration explosion or any other sort in their country. The City Council of Cambridge acted wisely and with courage earlier this fall when it called for bilateral disarmament. And tomorrow, Harvard professors and students will join the Union of Concerned Scientists in a day-long discussion of the problem. These signs indicate that we may sill have time to correct the error we made in not decided immediately to disarm after World War II.

Many valid objections to unilateral disarmament exist; for the most part, they boil down to one common idea. That without the deterrent offered by nuclear weapons, the Russians will be free to walk all over us in the world, and perhaps even in this nation. While we think the lessons of Vietnam and Afghanistan may show that chance is not as great as some think, we recognize the distinct advantages for all concerned of bilateral disarmament. As first steps, our administration should work for a verifiable ban on all test explosions and on flight tests of guidance systems. We should also adopt the freeze on new productions recommended by millions of Americans who have signed a petition directed at Congress (even if such a freeze is unilateral, our overkill capability will make it a safe plan for us to follow). And from that base the United States and the Soviet Union must work to reduce arms in both nations until none are left, and at the same time to persuade other nations with much smaller nuclear capabilities to go along.

The nuclear peace cannot be assured by the super- powers alone (though they are the only ones with arsenals large enough to wipe out the planet). Disarmament must extend to the other nations with nuclear capabilities, and the U.S. and Russia must use every economic, diplomatic and political pressure to insure that these nations too disarm.

Given the past history of arms talks, it is easy to despair about the chances for real reductions. But much of that failure is due to the idea that nuclear arms reductions can be pursued in the same manner and with the same urgency as other diplomatic goals. They can't. We must cease the games of atomic chicken that Haig has played in recent days. And then we must make disarmament our top priority, even if it means the sacrifice of some short-term hopes for a better world.