Moments after his earphones told him the countdown was over, the experiment successful, Donald Moore opened the silver-gray flash curtains that protected him from blindness.

At night, through his cockpit window, Moore felt as if he were looking into the face of an angry God. High over the Pacific Ocean the heavens were afire. The pilot, a young lieutenant just beginning a long Air Force career, gazed upward through an unearthly prism at an atmosphere ionizing in a blazing spray of deadly, radiant colors.

Moore was awe-struck. He couldn't conceive of fighting wars with the power on display above him. Deterring wars, perhaps. Fighting wars, never. He quickly concluded that the slogan of the Strategic Air Command--Peace . . . Is Our Profession--simply had to be more than Orwellian double-speak. That was 20 years ago, 1962, and mankind already had come a long, long way in the first 17 years of the nuclear age. It was in the early '60s, about the time of Moore's flight, that the world's doomsday fears crested for the first time in missile crises and test ban treaties. It was soon after that that the world's fears, too awesome to handle, too easily diverted by other worries and placated by treaties, began to drop into the underground of the mind.

And it was then that the secret nuclear world, almost unnoticed, began to evolve away from bizarre strategies of Mutually Assured Destruction to even more frightening strategies of limited wars. It was then that men began to seriously devise strategies for such wars, strategies that they believed made the balance of terror more credible but, by the '80s, made limited nuclear wars far more credible as well. Understanding the '60s, and the evolution of the following two decades, goes a long way toward understanding the new public alarm of the '80s.

The explosion Moore watched 20 years ago, in an operation code-named Starfish Prime, was a small one by man's new standards--a single megaton, more than 50 times the strength of the bomb that leveled Hiroshima. But by 1962, Strategic Air Command planes routinely carried 20-megaton bombs in their bays. And the Soviet Union, which had long since broken the nuclear monopoly so fearfully and naively held by the United States, had tested a 60-megaton hydrogen bomb.

There is no theoretical limit to the size of a thermonuclear weapon. In theory, if not in technical prowess, a single bomb could extinguish the entire earth. Vaporize it. Create a new asteroid belt between the planets Venus and Mars.

Starfish Prime was one of America's last atmospheric tests and an unusual one, at that. From lonely Johnston Island in the mid-Pacific a rocket carried the bomb 240 miles into space before it was detonated. A thousand miles away in Hawaii, people reported they could read a newspaper under the light of the man-made midnight sun.

In microseconds a surge of power burned out 30 circuits of street lights in Hawaii. High frequency radio transmissions were disrupted for 20 minutes in an area from Australia to Hawaii to San Francisco and for 40 minutes between Japan and the United States.

The phenomenon was known as electromagnetic pulse. Scientists were vaguely aware of EMP, an electric surge more than 100 times stronger than lightning. It is generated by all above-ground nuclear explosions and harmless to human beings. But after Starfish Prime the scientists concluded that a handful of high altitude explosions, perhaps a single large detonation, could burn out most communications in the United States.

Men were making other discoveries, too.

In Fort Wayne, Ind., Dr. Jack Geiger was collecting baby teeth. He hounded mothers, pestered dentists and sometimes offered bonuses that made the tooth fairy look like a piker.Geiger, a former newspaper reporter who had covered the atom spy executions of the Rosenbergs and then went into medicine and crusading, knew that radioactive fallout from open air testing was creating a worldwide health hazard.

Although government officials downplayed the risk, Geiger was far from alone in his fears. As early as the '50s mothers were marching against strontium 90, a radioactive pollutant that rises into the stratosphere and then falls slowly to earth, where it lands on grass, is eaten by cows and is retained in their milk. It is a bone-seeker and children are particularly vulnerable.

In the baby teeth, Geiger, Dr. Ralph Lapp and others found unusually large concentrations of the lethal substance.

Other men, in other ways, were struggling to deal with the nuclear dilemma.

Adm. H. G. Rickover already was well on his way to his dream of a nuclear Navy in which a single hidden submarine could carry enough megatonnage to destroy every major city in the Soviet Union.

In mid-1962, shortly after Starfish Prime, President Kennedy presided over the planting of Minuteman intercontinental missiles in the wheatfields of Montana, beginning an era in which delivery times were down to less than 30 minutes. "My ace in the hole," Kennedy said.

Soviet Premier Nikita Khrushchev vowed that his country would "bury" the United States and Americans were left to ponder whether to take that literally or figuratively.

Herman Kahn, the preeminent nuclear war theoretician, was churning out war strategy advice for American leaders in tracts and books with titles such as "On Thermonuclear War" and "Thinking the Unthinkable." In one he outlined a list of 44 stages of escalation, with names such as "Hardening of Position" and "Provocative Counter Measures." The 44th was "Spasm War."

In October, 1962, during the Cuban missile crisis, the world came as close as it ever has to the spasm. Missiles poised against missiles. Bombers poised against bombers. Megatons poised against megatons. Kennedy poised against Khrushchev. "We were eyeball to eyeball," Secretary of State Dean Rusk said later, "and they blinked."

But if the world relaxed and many Americans took a certain macho pride at the end of 13 days on the nuclear brink, both moods were misleading. President Kennedy's brother, Robert, later wrote the insider's view of the world's first trip to the cliff. His book, "Thirteen Days," was anything but reassuring.

Robert Kennedy wrote of tensions so great inside the White House that the president's closest advisers became exhausted and almost irrational, so tense they "almost seemed unable to communicate with one another," let alone communicate with the Soviets.

He wrote of his own feelings that "we were on the edge of a precipice with no way off."

He wrote of an inability to get messages through to Moscow in less than hours. He wrote of a president whose "face seemed drawn, his eyes pained, almost gray" in the final minutes as Soviet ships bore down on an American blockade in the Atlantic. "Was the world on the brink of a holocaust? Was it our error? A mistake?"

He wrote of a showdown whose outcome had moved beyond reach of the most powerful men in the world. "President Kennedy had initiated the course of events," his brother wrote, "but he no longer had control of them." The nuclear hair trigger was cocked and, presumably, it would have fired. The situation was, briefly, out of human control.

A year later, partly because of the strontium 90 in babies' teeth around the world, partly because of the sheer terror of the eyeball-to-eyeball confrontation over Cuba, partly because the superpowers wanted to prevent other nations from obtaining the bomb, the United States and the Soviet Union signed and ratified an open air test ban treaty.

Other nations--China, France, India, perhaps South Africa--continued to test in the atmosphere, for the bomb had become almost a talisman of power and prestige. The superpowers went underground. So did public worries, hidden in the recesses of the psyche just as the relentless nuclear tests were hidden in the recesses of the earth.

Geiger stopped collecting baby teeth and took up the civil rights cause. The next generation of marching mothers took up Vietnam, then pollution, then nuclear power plants.

"After the test ban treaty, people breathed a sigh of relief," says Roger Molander, who recently emerged from seven years in the thinking-the-unthinkable world of the National Security Council to form Ground Zero, one of the nuclear protest groups proliferating now as fast as the weapons. "They were scared silly by the Cuban missile crisis and then they just settled back down and forgot it again."

Even Kahn, the modern day Machiavelli proffering advice to nuclear princes, dropped out. The princes, and the generals at the war colleges and the Pentagon, stopped listening to the esoteric theories of the man who built 44 steps to a spasm nuclear war. He wanted to talk to them about how to turn off an accidental war, how to prepare for a limited war, how to handle a protracted war.

"We had a lot of ideas in the '60s, a lot more than the war colleges were interested in or people were interested in," Kahn says. "If you're an American military officer you get your promotions by being a good manager. Planning ability is the last thing people want from you. And, interesting enough, your nuclear war was just totally uninteresting. It was unthinkable basically."

So Kahn took up the study of futurism instead.

But if the protests stopped after the test ban treaty, if men and women with radically different views of the nuclear arms dilemma withdrew to psychic safety, the relentless world of The Bomb didn't pause. The world's supply of nuclear devices quietly quadrupled and sat silently poised and waiting.

Every day since the test ban treaty, men have sat inside the mountain in which Donald Moore works now, watching computerized radar screens that have misread reflections off the moon as incoming Soviet missiles, that have seen flocks of Canada geese over Labrador and misread them as Soviet bombers, that have set off false alarms when 47-cent computer chips failed.

Every moment of every day, while men fought in Vietnam and others worried about Red Dye No. 2 and Three Mile Island, the nuclear battle staff sat poised near the red phone and the yellow phone in the Strategic Air Command's underground command center beneath Omaha, Neb. The lights in the SAC bunker turn an eerie metallic blue at the first sign of trouble--real, false or practice.

Every minute for 21 years the flying command post, known as Looking Glass, has circled over the Midwest ready to take over the war if the eerie blue lights go out, as they would. The plane was named for the Looking Glass through which Alice stepped into Wonderland.

Every day bomber crews sit in alert bunkers beneath klaxons that go off, on average, twice a week to keep them ready for the real thing. Submarine crews move silently beneath all oceans, gyroscopes adjusting the trajectories of their missiles every microsecond so they are constantly trained on their targets, this one for Leningrad, that one for Kiev.

Every day, on 24-hour shifts, pink-cheeked young men in their early 20s, some of whom weren't born when President Kennedy was elected, take elevators deep beneath the American prairies to Kennedy's ace in the hole, the Minuteman intercontinental missiles. Two men to a capsule, sitting 12 feet apart, each with a key that, if turned together, would unleash more destruction than has been unleashed by all the warlords and princes in the history of the planet.

On the other side of the globe young Russians have similar keys, the youth of two great nations faced in a surreal standoff, holding each other and the world hostage. The American missilemen call their dug-in silos "the first good-byes," for they know their buried bunkers are the first targets of the young missilemen a half-spin of the planet away.

Theirs is the world of Mutually Assured Destruction (MAD), a world that many believe is the safest man can construct now that science has given us E=mc2, Einstein's theory that led to splitting of the atom.

The theory of the MAD world is that as long as both sides have those young men poised in bunkers, holding enough destructive power to annihilate the other, neither side would be mad enough to try. It is also called deterrence.

But many also believe it is a fool's world. To believe that the world can be held hostage, they argue, that men can face each other indefinitely with nuclear six-shooters cocked and aimed at each other's chests, is as delusionary as believing that E=mc2 can be wished away.

And so the the underground world of The Bomb has drifted methodically and quietly away from MAD and toward a new set of acronyms. LNOs for Limited Nuclear Options, and--in the wry, black humor that gave mankind MAD--toward a strategy called Nuclear Utilization Target Selection. NUTS.

Those theories mean the opposite of deterrence. They mean fighting nuclear wars, surviving them, winning them. Or, at least, constructing strategies that prepare for those possibilities.

In a way, the new theories are not new at all but merely a restatement of the dilemma that began on July 17, 1945, near Alamogordo, N.M., in the valley the Spanish called the Journey of the Dead.

The Army's Gen. Leslie R. Groves, military director of the Manhattan bomb building project, after all, had argued at the beginning that the atomic bomb was just a bigger bomb. Douglas MacArthur, in signing the peace agreement with Japan after Hiroshima and Nagasaki, disagreed, saying the atomic bomb had brought into question the "survival of civilization." Standing aboard the battleship Missouri in Tokyo Bay, with Japan humbled at his feet, he warned: "We have had our last chance. If we do not now devise some greater and more equitable system, Armageddon will be at our door."

But five years later, with the Chinese storming across the Yalu and driving his troops chaotically south in Korea, MacArthur suggested "laying a field of radioactive wastes--the byproducts of atomic manufacture--across all the major lines of enemy supply." The moral lines are drawn more finely during the fog of war, especially during defeat.

President Truman said publicly that atomic weapons might be used to stave off the Chinese. Under heated international pressure, he backed off, but the White House issued a statement saying, "The use of any weapon is always implicit in the possession of that weapon."

During the Cuban missile crisis, President Kennedy was horrified to learn that battlefield commanders apparently had authority to launch nuclear weapons against the Soviets without his direct approval.

Kahn acknowledges that the United States looked at the possibility of using nuclear weapons in Vietnam but abandoned the idea "because there were no targets there" and "the means were not proportionate to the end."

The United States has always kept its options open, and has at least looked at the use of tactical nuclear weapons in every major crisis or war since Hiroshima and Nagasaki.

By 1974, the policy was changing publicly, when a presidential directive gave the United States the targeting options to fight a variety of limited wars. About that time the so-called counterforce strategy--a limited war in which the superpowers might selectively attack missile silos, bomber bases and submarine ports--became fashionable.

One episode in the mid-'70s tells a lot about limited nuclear wars and why the public, carefully shielded from many nuclear realities, is on the march again.

In 1974 Senate testimony, Defense Secretary James R. Schlesinger described a counterforce attack by the Soviets that would result in "relatively few civilian casualties." Under questioning, he said the number would be "15,000, 20,000, 25,000--a horrendous event, as we all recognize, but one far better than the alternative."

American missile silos are located primarily in the sparsely populated prairies. But even towns like Great Falls, Mont., which is surrounded by Minuteman silos and might absorb hundreds of megatons in such an attack, have populations over 25,000. Bomber bases are on the edge of towns such as Sacramento; Shreveport, La.; Spokane, Wash., and Utica, N.Y. The new Trident submarine base is on the outskirts of Seattle, a city with a population of more than 1 million.

Schlesinger's casualty estimates for a counterforce war were greeted with incredulity. Months later he issued revised estimates of 800,000 dead, with another 700,000 dying later from radiation. Congress did its own study and concluded that the deaths would range between 2 million and 20 million. The Pentagon eventually agreed the congressional figures were more realistic.

Still, it took the 1980 presidential campaign to bring the nuclear dilemma to the forefront of the public mind again.

Early on, George Bush created a minor storm by stating publicly that he believed a nuclear war was winnable. Ronald Reagan spoke of an arms buildup that could cost more than a trillion dollars, arguing that the United States faced a "window of vulnerability" to nuclear attack in the '80s and that the Soviets believed a nuclear war could be won.

When the Soviets invaded Afghanistan, Jimmy Carter drew a line in the oily sand of the Middle East and said we would defend against any move into the Persian Gulf. Within days, the Pentagon leaked a report that the only way the United States could defend the far-off gulf was with nuclear weapons.

By mid-1980 Carter issued Presidential Directive 59, a top-secret targeting policy that further institutionalized America's ability to fight selective nuclear wars. Then came the election, and a return to saber-rattling rhetoric.

It was good-bye to the psychic numbing that kept the public distracted from man's greatest dilemma.

Jack Geiger, as a spokesman for Physicians for Social Responsibility, is traveling from city to city telling what a nuclear explosion would do your home town. A Starfish Prime, one megaton, would kill 780,000 persons in San Francisco and seriously injure 382,000 more, he told the Bay area. There would be few physicians and almost no hospital beds for the wounded.

Herman Kahn, for there are many sides to the dilemma, was back in the business, relieved that the psychic numbing was over so that strategists, as well as citizens, would think the unthinkable once again. He is convinced that the world is a far more dangerous place now than it was when Americans buried their worries almost two decades ago.

The last of the Kennedy brothers, Sen. Edward M. Kennedy (D-Mass.), is back on the podium pushing for a freeze on production of nuclear weapons.

Some of this worries Roger Molander. He worries that the freeze movement is a palliative, somewhat like the test ban treaty, and that after a spurt of public energy, "we will drift back to business as usual just as we did in the past 20 years."

Molander sees no easy answers, no quick technical solutions. He comes at the dilemma from a far different direction than Kahn. But he, too, thinks the world is more dangerous now than it was. It was after the test ban treaty and during SALT treaties and talks and detente that the nuclear behemoth quietly grew.

Still, it is that point--that the world is so much more dangerous--that could make the '80s different, for better or worse. Far more than at any previous time, serious men are questioning whether civilization, or even the species of man, could survive a full-fledged nuclear war. On this question, as on almost all questions involving the nuclear age, men disagree. But the disagreement is not wide.

"We have examined that over many, many years," Gen. David C. Jones, chairman of the Joint Chiefs of Staff, testified last winter. "There are many assumptions that you have as to where the weapons are targeted. Clearly, the casualties in the Northern Hemisphere could be, under the worst conditions, into the hundreds of millions of fatalities."

In his farewell testimony on Capitol Hill, Rickover, the man who had placed thousands of megatons beneath the seas, startled senators with an incredibly bleak look at the future. "The lesson of history is that when a war starts, every nation will ultimately use whatever weapon has been available," the outspoken admiral said. "I think we probably will destroy ourselves. So what difference does it make? Some new species will come up that might be wiser."

Kahn calls Rickover's testimony his "senile" farewell address. Back in 1948 when he first began studying the unthinkable, Kahn acknowledges he had "a little shudder" that "the atom bomb meant the end of civilization." He says his first study convinced him the "atom bomb just can't do it."

But even Kahn, in his intellectually ruthless approach to the problem, quickly adds: "Now the H-bomb, which is 1,000 times bigger, raises different issues."

Geiger just shakes his head. "The point is we don't know what we are playing with," he said, "and we won't know for sure until it's too late. Survival is social as well as physical. Assuming the best, the handiest person to have around afterward is going to be a medieval historian to tell us how the world worked in the 9th century."

Donald Moore is a colonel now, nearing his first general's star, and he has watched the weaponry and the strategy of his business change over the years. But he never has forgotten his youthful ringside seat at Starfish Prime.

"Anybody who has seen a nuclear explosion has a changed perspective forever," Moore says. "All this talk about limited nuclear wars and survivability. I don't know. I don't know if we could survive one of those going off . . . ."

Moore's voice trails away, leaving an uncertainty about whether he means civilization or his place of occupation. He works now in America's nuclear warning post near Colorado Springs--the strange hollowed-out cavern dug into the solid rock of Cheyenne Mountain. The command post is covered with 4,000 feet of granite. It would not survive a direct hit by a modern Soviet missile.

No one knows what would happen in a nuclear war--whether the missiles would be as accurate as claimed, whether communications would be destroyed by the effects seen in Starfish Prime, whether the atmosphere's ozone level would survive.

Last month, in a top-secret White House operation code-named Ivy League, the nation's top officials put America's nuclear arms apparatus through a mock Soviet attack. It began with use of tactical nuclear weapons fired on an American ship in the Atlantic and a chemical warfare attack on overseas U.S. troops.

For four days, the situation escalated. With stand-ins acting as president and vice president, the officials made "real-life" decisions that included responding in kind with tactical nuclear weapons and placing the "vice president" aloft in the National Emergency Airborne Command Post, the refitted 747 known alternately as the Doomsday Plane or the Flying Fuehrer Bunker.

On the fourth day, Cheyenne Mountain reported that the Soviets had launched 5,000 megatons at the continental United States. Five thousand megatons represents 250,000 times as much explosive power as the bomb dropped on Hiroshima.

The vice president was aboard the 747. The president was vaporized in the Situation Room in the basement of the White House. After the exercise, President Reagan thanked the people at the warning posts and command centers and reported to them that the system had worked well.