The middle of a Soviet peace offensive is traditionally the season of silliness about the Russians. London is only now recovering from its swoon over the Gucci comrades, those adorable Gorbachevs.

In America the nonsense is more esoteric. It has to do with a current Gorbachevian preoccupation: President Reagan's Star Wars proposal for nuclear defense.

Gorbachev does not like it. He argues loudly that it makes arms control impossible. At home, that idea is given wide currency by the Gang of Four ex-strategists (George Kennan, McGeorge Bundy, Robert McNamara and Gerard Smith) in an influential Foreign Affairs article entitled "Star Wars or Arms Control." The president must choose, they warn; he can have one but not both.

The trickle-down effechas begun. CBS News's "Face the Nation" led off its program this week thus: "Arms control talks with the Soviets could be in trouble because of Reagan's support for Star Wars."

This idea, soon to achieve the status of conventional wisdom, is exactly wrong. In fact, arms control talks with the Soviets are in existence because of Reagan's support for Star Wars.

The Soviets, who walked out of two sets of missile talks a year ago, have returned to the table. Why? Their ultimatum to the United States -- to remove its missiles from Europe -- has not been met. Internally, nothing has changed. Soviet leadership is at least as much in transition today as it was a year ago. Yet one thing is new. Reagan has turned Star Wars from an idea into a $26 billion program. The Soviets are desperate to stop it.

The puzzle of the day is: Why? Study after study shows that the president's dream of a defense that renders nuclear weapons "impotent and obsolete" is a fantasy. Why are the Soviets afraid? So afraid, in fact, that they have reversed field, swallowed their pride and conditions, and returned to Geneva.

What do they know that we don't?

They know that for a nuclear defense to "work" can mean two things. Many of the president's critics (and supporters) have been so mesmerized by his vision of a nuclear-free world that they have overlooked one of those meanings.

In his March 1983 speech, Reagan sold Star Wars as a cure for deterrence. He still sells it the same way. Only last week he said again that any defense that rests on the threat to kill millions of people (i.e., deterrence) is immoral. He wants an American defense to rest instead on a technological shield.

It is a fraudulent sale. Politically brilliant, perhaps -- Reagan gets to dish the freezeniks by coopting their horror of deterrence -- but a fraud nonetheless.

No defensive system imaginable can protect populations. That is the conclusion not just of congenitally anti-nuclear (and anti-Reagan) groups such as the Union of Concerned Scientists. One of the first Pentagon-commissioned studies, the Hoffman report of October 1983, makes the same point, albeit obliquely. It concludes that in the "intermediate" term -- a delicate way of saying "your lifetime and mine," without offending the president -- we must think of something other than population defense.

The reasons are simple. Such a defense must be unimaginably perfect. (A system 99 percent effective allows 100 bombs through, and 100 bombs can mean the end of the United States.) Offensive countermeasures are easy and cheap. And -- the clincher -- the most perfect Star Wars defense does absolutely nothing to stop bombs delivered by cruise missile or bomber. QED.

A defense meant to protect populations won't work. But Star Wars can work in a second way, a way that holds little appeal for American citizens (hence Reagan's silethe subject), but great interest for Soviet strategists: It could protect weapons. A Star Wars system only partially effective could protect America's retaliatory (second-strike) capacity, because to retaliate effectively only a fraction of one's missiles need survive.

The Soviets are frantic about this prospect because they have invested hundreds of billions of dollars in a huge first-strike force of SS17s, 18s and 19s. An imperfect Star Wars designed to defend weapons, in effect and unilaterally, thins this force. It closes what Ronald Reagan once called the window of vulnerability. It is arms control by (American) diktat.

Faced with a challenge to the most important and most threatening element of their nuclear arsenal, the Soviets have two choices. One is to compete. Now, adapting an offense to defeat a city defense is easy. You have only to find a way to get, say, 10 percent of your warheads through. But to defeat a defense of missile fields, you have to get a very high percentage of your warheads through. An independent (and critical) study commissioned by Congress' Office of Technology Assessment estimates that overwhelming such a defense could require 100 times the effort needed to overwhelm a city defense. Maintaining a potential for a disarming first strike thus becomes an exceedingly difficult and impossibly expensive proposition.

The other Soviet alternative is to negotiate. Hence Geneva.

Far from being a threat to arms control, Star Wars, by offering a unilateral American alternative, is an inducement. It has already coaxed Gromyko to Geneva and Gorbachev to declare that his side is willing to negotiate radical reductions in offensive arms -- once Star Wars is dealt with, of course.

Which suggests an American agenda for the New Year. The Gang of Four, which wants Congress to kill Star Wars, takes a vacation from advice-giving. Congress leaves Star Wars to the president. The president renounceshis fantasy of repealing deterrence and accepts the reality of an imperfect (deterrence-enhancing) Star Wars. Secretary Shultz carries this version to Geneva, and deals.