A boulder in the road

President Reagan and his severest critics in the peace movement share one thing in common: a preoccupation with nuclear weapons. For both, strategic nuclear war is so overwhelming a threat that the clear and present dangers to peace aren't attracting the attention, the imagination, or the resources they deserve.

Whatever their intentions, both are busy scaring the American people into puzzled paralysis, conjuring up futuristic scenarios that erode the political energy we need to address the more proximate threats to world security (a synonym for US national security these days): a dozen conventional wars with no peaceful settlements in sight, the widespread degradation of the global environment, a probable breakdown in the world monetary nonsystem, the disintegration of governance in more and more Lebanons and El Salvadors - and the rising tide of international terrorism quickened by all of the above.

It's as though the war gamers and the peace demonstrators are standing together in front of a huge boulder, a grotesquely large ticking bomb planted in the middle of the path to peace. They can't move it aside, and they can't seem to see around it to plan the rest of their journey. Maybe it would help if they were both able to think of this ticking menace as a probable dud.

Nuclear weapons have turned out to be ''ultimate'' in an unanticipated sense of the word. No nation's military planners have been able, in 38 years, to think up a way to use such huge explosions with such pervasive aftereffects in ways that are clearly advantageous to their side. This may now be the most important thing about nuclear weapons in the 1980s: that they are militarily unusable.

People who study deeply the effects of using nuclear explosions seem to conclude it's next to impossible to construct a nuclear scenario in which the ''solution'' isn't worse than the problem.

I had this experience myself when I worked for four years (1965-69) as the US representative on the North Atlantic Council (NATO's political board of directors). As a practicing war gamer, I failed to figure out how either the Soviets or our European allies could believe that the US president would breach the nuclear threshold in response to a conventional attack in Western Europe.

Henry Kissinger said something like this in a London speech not long after he left office as secretary of state. Adm. Noel Gayler has frequently spoken about the unusability of nuclear weapons since he left off being commander in chief of the US Pacific Fleet.

Robert McNamara told an Aspen Institute arms control seminar the summer before last that he (and he thought the two presidents he served as secretary of defense) would under no conceivable cirumstances have given the order to fire a nuclear missile.

The notion that nuclear weapons are unusable because suicidal has only been reinforced by recent research on their effects. The gathering of scientists in Washington, organized by Carl Sagan and others, served to highlight the biological consequences of nuclear war. One point was brought out that puts a new twist on the argument about nuclear unusability.

Deterrence, we have been saying for three decades, consists in the leaders of each superpower being credibly uncertain about what the leaders of the other superpower would do if. We haven't been paying much attention to the contingency that a major first strike might in itself wreak unacceptable damage on the attacker whether the other side retaliated or not. If on further study this boomerang effect shows this clearly enough, it might be that the deterring ''second strike'' is actually contained in the first strike. Such a self-inflicted second strike isn't ''mutual suicide.'' It's suicide.

Just think what this perception, widely advertised and accepted, would do for the stalled US-Soviet nondialogue. It is the ultimate argument for the ultimate unusability of strategic nuclear weapons. Would the Soviet leaders act on such a perception if the American people (and their government) make it the basis of US national security policy? Well, they seem to have followed each US twist and turn in arms-racing with a time lag of three to five years. Is it inconceivable that they could also come to understand the principle of the boomerang?

In the old Western novels people were sometimes hogtied in a fashion that strangled them if they struggled too hard to escape. Are the nuclear superpowers with their big strategic weapons now in a similar fix? And if so, is this maybe good news for humankind? Could we then get our eye back on the ball - the whole globe, not only the strategic nuclear threat to it?