Boston — Parable: Duke de A. is in his castle writing to his armorer. That grizzled artisan has just sent word from his smithy - the R&D lab of his day - that he has discovered a new way to harden the steel in milord's shields. If field tests confirm, the duke's forces will be able to sleep snugly, knowing that the most fearsome assault from the Duke de B.'s archers will be deflected harmlessly by the new shields.
But over in the Duke de B.'s castle, the ducal arms maker has heard intelligence reports about the new shield alloy. He is working furiously to provide de B.'s bowmen with new super-penetrating arrowheads.
Back in de A.'s castle six months later, the stalwart duke talks to his local da Vinci about a new tactic for neutralizing the new armor-piercing arrows. He will surround his castle with a wall of flames 100 yards outside the moat. The new super-arrows will have their guide-feathers singed. Shorn of their feathers, the enemy missiles will lose aim and momentum against the defenders' shields.
Castle de A. and Castle de B. are locked in an arms race that provides an uneasy measure of deterrence. But the villagers in the market town of Vegetable-Marrow-Under-de A. are not reassured. Duke de A. has offered to sell them his new hardened shields. Duke de B. has threatened them with his shield-piercing arrows. They see trouble from any move they make. De B. can pierce their shields. The Castle de A. cannot maintain a ring of fire far enough out to protect them. If they try their own ring of fire, the singed arrows, coming from close range, will fall haphazardly and lethally among the villagers. And they feel that they can't afford shields or the fire-ring, anyway, without ruining their economy.
And so it goes with arms races....
The foregoing is a reasonably close facsimile of the problem facing the United States and its West European allies as they debate whether to invest heavily in President Reagan's strategic defense initiative, the so-called ''star wars'' missile defense system. I've recently discussed the question with professional tactical planners from both sides of the Atlantic - advisers to Western alliance leaders and scholars of both armament and disarmament. What follows is an attempt to explain the gap between US star-wars enthusiasts (de A.'s armorers) and West Europeans, who believe - rightly or wrongly - that they live in the village outside the castle walls and moat. It is also an attempt to find a common ground to keep the Western alliance intact on this major issue of the next decade and perhaps next century.
First, let's look at the essentials.
* What the Reagan administration called for last winter is not the building of a star-wars defense but a speeded program of research to discover how to build a shield against incoming intercontinental ballistic missiles and their warheads.
* That research program would concentrate on laser, particle-beam, computer-guided killer missiles, and other defensive weapons that would intercept opposing nuclear missiles at three stages: while they are still ascending under initial rocket boost, in midcourse, and, as a last resort, after their many warheads have separated and are descending on targets.
* The research would cost a lot of money. Building a complete system, vastly more.
* Even the most vigorous backers of the star-wars concept don't argue that it would be penetration proof - only that it could be made very effective. In so protecting the bulk of American targets, they further claim, it would provide an unusually effective deterrent against missile warfare ever breaking out.
* Opponents say that during the long research and installation period, the East-West deterrence balance would be extra unstable and Moscow might be tempted to launch a preemptive strike because it felt a US missile defense was the prelude to an American strike.
* Precision-guided defensive weapons might make possible extensive use of conventional (nonnuclear) missile-killers. This still-hypothetical possibility of the triumph of nonnuclear defense over nuclear offense has attracted some unexpected supporters for star-wars research from the antinuclear camp.
* Some military strategists who are not otherwise squeamish about lethal weaponry are nevertheless dead set against a big star-wars program. They fear its cost would be such that conventional weapons programs, maintenance, and training would be shortchanged. This particularly worries European generals, who are concerned that NATO would be weakened in relation to the Warsaw Pact over a long period of time while star-wars research and testing soaked up money.
The image that tactical experts often use in discussing the subject is a space-age version of the medieval parable used earlier. It pictures the US protected by a ''bubble'' of high-technolgy weapons. The Soviet Union, playing catch-up technology, then constructs its own (perhaps less-sophisticated) bubble.
That is where many Western European strategic thinkers begin to see themselves as the village caught in between bubbles. They feel that shorter-range, lower-trajectory nuclear missiles on both sides of Europe would be much harder to defend against. And that therefore Europe would be the battleground. Some also worry about ''fortress America'' thinking arising in the US if a star-wars defensive bubble proves buildable.
American proponents of star-wars research argue that a more nearly impregnable America is all the more able to be a staunch defender of Europe. Furthermore, they assert, if the West doesn't use the new missile defense technology as it becomes available, Moscow will.
American opponents of star-wars research make several vigorous counterarguments. One is that a cheaper way to offset a Soviet missile defense would be to increase research on so-called ''penetration aids'' for Western offensive missiles. This would be the better-arrowhead approach. Other opponents argue that the Western democracies simply can't convince their voters to approve an expensive new program when there is high unemployment in Europe and defense budgets are already large. Still others are concerned about the instability that would arise if the current nuclear balance of deterrence is disturbed. They say the new course would make arms control talks even more difficult than at present.
Is there a sensible course for Western statesmen to steer when the views of their strategic advisers vary so widely?
First, the political leaders need to know more. So a modest level of funding for American star-wars research (shared with allies) is needed. Probably a yearly level about half what the Reagan administration proposes would suffice. Without such research there will be three gaps in Western knowledge.
1. We won't know for sure whether it may indeed become possible for nonnuclear defensive weapons to become dominant over nuclear offensive weapons. Past experience makes the experts skeptical. But exact knowledge, not conjecture , is needed.
2. The West won't know what Soviet researchers and weapons planners may discover and develop unless it explores star-wars capability.
3. Planners won't know whether developing defensive weapons or new penetration technology is the more effective, cheaper route to follow in deterring the use of offensive nuclear missiles.
Second, any modest program of star-wars R&D must be accompanied by a stepped-up exploration of arms control possibilities with the Soviets. Without such a vigorous and genuine effort to talk, several risks could quickly grow: risk of a debilitating new arms race, risk of a prolonged imbalance in deterrence, and risk of new divisions in the Western alliance.
The aim of all this chalk talk, whether about arrows or missiles, is to allow everyone to sleep more confidently and carry on real life with a feeling of safety. Some theorists claim people can only do so if they trust their weapons. Others argue that people can only do so if they trust their knowledge of their opponents' aims. Both are needed. And trust among opponents at any time in history can only come if there is a recognition that they are part of a larger brotherhood on the same globe, in the same creation. That has to be the underlying feeling in any approach to arms talks or domes of defense. It was true in the historic realm of arrows and shields. It is true in the history we are about to write in this new phase of the atomic era.