The president thinks it would be wonderful if we could find "a security shield that will . . . render nuclear weapons obsolete." It would indeed. But is this anything more than fantasy?

History has repeatedly shown that one side's technological advance drives the other side to try to counter it. When the Soviets deploy radar- guided air defense missiles, we learn to jam the radar. When the Soviets modernize their radars and increase their power, we learn to fly below them. When the Soviets deploy look-down radars on air defense interceptors, we change the design of our airplanes to pass "stealthily" and invisibly through them. We create clutter to spoof their radar, and they design computer-assisted radars to separate clutter from the real targets. Et cetera, etc., etc.

The same kind of technological chess match is bound to accompany the effort to create a "Star Wars" missile defense. I recently asked Gen. Charles Gabriel, chief of staff of the U.S. Air Force, if we were willing to have our offensive nuclear forces rendered obsolete by a Soviet defense against our ballistic missiles. His answer was: not until we are assured that we have "a perfect defense" -- which means never, since even proponents of "Star Wars" don't claim that a perfect defense is feasible.

I also asked Gabriel if it was true that, while we say we hope we can lead the Soviets in the direction of defensive systems, we are also working simultaneously on measures to defeat a Soviet "Star Wars"-type defense. He admitted that we were.

When I asked if it weren't inconsistent to work on countermeasures to a Soviet ballistic missile defense at the same time that we hope that they will perfect such a system, he said: "Yes, sir. We are facing the real world."

In other words, our military leaders are going about the normal business of measure/countermeasure. But the general's realism is in stark contrast to the president's illusion.

Since the dawn of the nuclear age, every other American president has believed that the only way to bring about a more secure world was to seek to negotiate limits on nuclear weapons. The Anti-Ballistic Missile Treaty was one of the most successful efforts to restrain the arms race by banning major departments of defensive weaponry, both on earth and in space. A "Star Wars" system couldn't be deployed without negotiated modification of, or unilateral abrogation of, that treaty. Will the Reagan administration start a new arms race event or are they operating under another illusion that deploying some new "Star Wars" system would be consistent with the present structure of arms control?

Secretary of State George Shultz recently told the Senate Armed Services Committee that we would seek to negotiate modification of the ABM Treaty before we deployed such a system. President Reagan also said so in a joint statement with British Prime Minister Margaret Thatcher.

But while the president has said that "whether 'Star Wars' is deployed" is a matter for negotiation with the Soviets, his secretary of defense said the opposite last January on "Face the Nation": "I am ruling out the possibility of giving up a strategic defense either in the research stage or, if it becomes feasible, in the deployable stage. . . ."

Our allies watch nervously, and the U.S. public wonders who speaks for this administration on the question of whether deployment of space-defense weapons is negotiable.

Congress also wonders. We are asked to appropriate $3.7 billion this year for "Star Wars" research, $2.3 billion more than last year's level, a staggering increase for a program with an uncertain goal.

Common sense should tells us:

1."Star Wars" may not be technologically feasible.

2.We don't know, with any confidence, the total cost of any such system.

3.If it is feasible, it may well be countered by overwhelming it with additional tens of thousands of offensive missile warheads, or by ASATs, or by redesigned Soviet missiles, or by ASATs, or by redesigned Soviet missiles, or by decoys, or by the Soviets' own "Star Wars" system.

4.Deployment of defensive weapons will lead to a defensive arms race, and the only way to achieve a more stable world is through arms control -- not unrestrained arms development.

5.Technology doesn't ensure safety -- it only ensures more advanced technology.