THE BOMBERS CAME during the day, the shells at night. Mother always insisted on going to the cellar at the first sound of approaching menace. But for a 15-year-old, an only son destined to be an art historian, perhaps a musician, the war was an annoyance. He insisted on staying above ground, in his room, where he could read books on Byzantine history he borrowed from his father's friend, a scholar.

One night when the shelling got heavy -- the Nazis were getting near -- the boy consented to go to the cellar. Next morning, when it was quiet, the family ventured upstairs and found a gaping hole above the boy's bed and a 4-foot-long shell, unexploded, on the floor.

All this happened in Warsaw, in the first days of World War II. Today the boy is ensconced in the Executive Building, the battleship-gray Victorian palace that towers over the White House. He is the National Security Council's senior adviser on the Soviet Union and Eastern Europe. He regularly briefs the national security adviser and, on occasion, the president.

He tells us not to be afraid of nuclear war but to learn to live with the reality of its threat.

He says the current probability for nuclear war is roughly 40 percent. But, he adds with a reassuring smile, nuclear war won't happen, provided we build up our strength some more. His strategy, which he says reflects official thinking, is a winnable nuclear war. It is the fear of defeat that deters the Russians from attacking, he argues, and a threat of our victory may perhaps force the next generation of Kremlin leaders to negotiate real disarmament. "That's the only hope," he says.

He is Richard Pipes, 5 feet 8 inches tall with a scholar's stoop. To him, disarmament is as utopian a concept as perfect happiness or universal peace, and war is a risk as long as there are sovereign states. "We are just catching up with their arms race," he says. "They've been racing alone for years. There was only one time in Soviet history that they engaged in massive appeasement: from August 1939 to June 1941. That was because Stalin was enamored of Hitler. He thought he understood Hitler -- that Hitler was the same cunning gangster he was. Stalin believed it would make no sense for Hitler to attack Russia after Stalin had given him carte blanche to conquer Western Europe."

Pipes is the one Harvard professor in the Reagan administration, and one of the world's leading authorities on Russian intellectual history. What has aroused controversy is his politics -- his application of his scholarship. Time magazine recently described him as the hardest of the hardliners. Five years ago, The Washington Post's Stephen Rosenfeld began a column by saying, "For rank hysteria in scholarly garb, it's hard to top Harvard Prof. Richard Pipes' Commentary article, 'Why the Soviet Union Thinks It Could Fight and Win a Nuclear War.' "

State Department officials reassure reporters -- and foreign diplomats -- that his impact is limited because, as one veteran Foreign Service hand put it, "Pipes is an ideologue who tutors the White House." He is the first to admit that he is not a politician. Unlike Henry Kissinger and Zbigniew Brzezinski, Pipes is not a master in finessing his ideas through the bureaucracy. But from the beginning, the Reagan administration has adopted as its doctrine Pipes' notion of a Soviet Union striving to achieve military superiority.

Describing himself as a realist who tells it as it is, Pipes calls his opponents illogical. He says it is his duty as an intellectual to produce ideas; it is up to the politicians to market them.

Pipes is a lively lecturer who enjoys exhorting his audience to face up to the cruel reality of a Soviet empire bent on expansion. He is one historian who is also a political activist. A charter member of the neoconservative group writing for Commentary magazine, he headed in 1976 a "Team B" commissioned by George Bush, then CIA director, which provided a much grimmer view of Soviet capabilities and intentions than a "Team A" of the government experts.

Though a Democrat, he joined Ronald Reagan's transition team; his critics and friends thought he had hopes of following in the footsteps of his colleagues Kissinger and Brzezinski.

One of his recommendations was to create the post of an undersecretary of state in charge of the communist world. Fearing that Pipes might want the post for himself, the Foreign Service bureaucracy killed the idea. Alexander Haig would not have him in his State Department. National Security Council chief Richard Allen inherited Pipes, but ordered him not to speak to the press after an early interview which put on the world's front pages his assessment that the Kremlin must choose between reforms and war.

To Pipes' great surprise, William Clark, who succeeded Allen last January, has unmuzzled him. This story is based on an interview that began in his office and continued over a lunch of veal parmesan and Valpolicella.

"Once we upgrade our nuclear deterrent to the point where it is credible, then the Soviet strategy of winning a nuclear war becomes incredible," says Pipes. He refers to what is known as "counterforce": a shifting of targets from primarily civilian to primarily military. "We used to believe," says Pipes, "that if you threatened the Russians with the destruction of 100 to 200 cities, that would be a sufficient deterrent. This notion is being abandoned. First of all, it is a highly amoral doctrine, this idea of massacring civilians for the decision made by their dictators, their military leaders, over which they had no influence.

"Secondly, the Russians have made it clear they would not accept such rules of the game by developing a civil defense program, air defenses, and so on. So they really don't accept the doctrine of Mutual Assured Destruction. Because they develop defensive systems against nuclear war, indicates that they don't believe in Mutual Assured Destruction. Therefore our strategic problem has changed because we cannot rely, for both moral and military reasons, on the threat of destroying civilian populations as the principal deterrent.

"The worst thing to do is to be afraid. That way you reinforce the premise of the aggressor. As Clausewitz somewhere says, all war assumes human weakness and is directed against it. And in conflict, fear is the greatest human weakness. The instant you cease to be afraid, force loses a great deal of its effectiveness. This is especially true of nuclear weapons which are meant as much to intimidate as to destroy.

"If we could invent a ray that we would spread above the United States and Europe, creating a kind of an invisible atmospheric umbrella which no Soviet missile could penetrate, because the moment it would hit it would disintegrate, then the whole psychology would change. Fear would disappear and people would not tolerate a lot of things that the Russians are doing, which they are now tolerating. Especially in Europe, which is so frightened."

Pipes throws his hands in the air and shrugs his shoulders when asked about a possible scenario for a nuclear war. It can happen anywhere, he says. Nuclear war is, after all, an extension of conventional war. The Russians may launch it, or, more likely, an irresponsible government, a latecomer to the nuclear club.

But it won't happen, Pipes adds with the easy smile of a life insurance agent. "If we are allowed to beef up our defenses and do all the things we are planning on doing, our retaliatory power will be such that the risks of nuclear war will diminish," he says. "We're not going to initiate a nuclear war. If we are able to frustrate Soviet strategic plans, I think the chances of nuclear war will decrease much more. Because there will be a true standoff."

Pipes acknowledges that it is a global game of chicken, and he chuckles as he agrees. "What Hitle mar got in the 1930s he got because he was perceived as strong and bold," he says.

But he tenses up when he insists that the number of nuclear warheads "matters enormously." He is alarmed that though the number of American and Soviet nuclear warheads are roughly equal now, the Soviets have five times the American nuclear megatonnage -- they prefer building bigger bombs -- and they keep adding to their arsenal at a much faster rate than we do. He argues that it is not just the perception of strength that matters but the actual strength in megatonnage plus accuracy in delivery. "A combination of a lot of MIRVs, high yields and improvements in accuracy, creates a situation where in a few years they can knock out the great bulk of what we have and leave us with a very small force to strike back," he says. "Therefore we would like to develop a missile which, by being mobile, could not be knocked out with two warheads but would require many more. The original idea was that the aggressor be required to use 20 or 30 warheads to be sure of knocking out a single MX; this would deplete his forces. Secondly, by MIRVing the MX -- giving it eight or 10 warheads -- we also buy a tremendous counterforce capability."

He says that the Reagan administration has scrapped MAD -- Mutual Assured Destruction -- which was the nuclear doctrine that let us sleep at night for the past 30-plus years. Our new doctrine is a kissing cousin of what Pipes has identified as the Soviet doctrine: The losses, while terrible, can be scaled down with careful planning, and our determination to win a protracted conflict is the best way to keep the enemy from attacking.

Thus the old balance of terror immobilizing both parties is no more. Judging from the vantage point of today's "worst-case" scenario, it seems that our strategists adopted that doctrine because it was as necessary a public illusion as the superiority of the American auto industry or the possibility of making friends with the Soviets one day.

MAD was a way of telling ourselves that nuclear war won't happen; it can't happen. No sane person would pull the nuclear trigger. The Bomb will just go away, and irresponsible governments will never get hold of it. The unthinkable simply won't come to pass, so why think about it? Joe, say it ain't so, we said, and our leaders complied.

Pipes' thesis is that for the Russians, the distinction between conventional and nuclear war is not anywhere as sharp as it is in our thinking; they think of warfare as a continuum beginning with low-level diplomatic pressure, going through subversion, terrorism and proxy war before reaching conventional war, limited nuclear war and, finally, all-out nuclear war.

"We used to think that the notion of nuclear war as mutually suicidal and by definition unwinnable was the only conceivable strategy," he says, "and assumed that the Russians shared it. Then the evidence began to come in, primarily through the Russian buildup, after 1969, when the Russians attained strategic parity, that they don't share this point of view. So it isn't that we view nuclear war differently but we have come to appreciate that they view it differently."

Welcome to the stark world of the winnable nuclear war. In this comfortless landscape, liberals are pessimists because they are sensitive to the possibility that humankind may be nearing its end, and conservatives are optimists because they believe that another escalation of American nuclear power will assure that the Soviets won't attack.

"As you move toward the liberal end of the political spectrum you will find more pessimists than on the conservative end," Pipes says. "As a rule, liberals tend to be more emotional than analytic; they seem to derive a great deal of satisfaction out of indignation and exhortation whether these activities have any bearing on reality or not. They love to exclaim 'Stop this madness!' and to predict that mankind is doomed. Conservatives are more skeptical, more restrained in their emotions, more rational. There certainly is less uncontrolled anxiety about the possibility of nuclear war on the right than the left."

Pipes says the reason for his "moderate optimism" is simple: "There is no experience with nuclear war, and generals are not hasty with using weapons they have no experience with. You can plan all you want for the contingency of a nuclear war and you can test all your weapons, but they have never been tested in combat. An argument can be made that the existence of nuclear weapons has in fact prevented war in the last 35 years. There were several occasions when war might have broken out between us and the Russians, were it not for the presence of nuclear weapons, which have a very intimidating effect on their possessors. War has become so frightening that people are not so likely to launch it. In World War I, expectation was that it would be over in eight to 12 weeks, because you knew roughly what weapons you were disposing of, whereas today more than in any period in history, I think, you'd be facing unpredictable, imponderable situations, and that has a very inhibiting effect on the military."

Pipes says he was astonished to read a survey which claims that 70 percent of today's undergraduates expect to die in a nuclear war. "I think the probabilities are greater they may die in a car accident," he says. He remembers that during the Cuban missile crisis, in 1962, he thought it was disgraceful when half of his class, in Russian history, didn't show up. "You should carry on," he remembers saying. "What were they doing at home? Chewing their nails?"

He cites a recent study of people who lived to be more than 100 years old. "They took life as it is," he sums up the study. "They didn't worry. One should seek to cope with one's problems, not to worry about them. For my part, I prefer to work, through speeches or publications and now in the government, to create conditions that will lift the threat of nuclear war. Worrying as such is useless, it is an escape from responsibility."

The Russians emphasize not to be afraid of nuclear war, he says. Their slides and civil defense manuals show happy people marching to shelters. Their shelters are for the elite, he notes, which of course wouldn't do in this country. But the Russians also recommend simple devices, such as foxholes five to six feet deep, with three feet of branches and dirt on the top, which they say is good enough protection against a blast and radiation.

In America, Pipes says, large-scale evacuation of civilians is virtually impossible because 98.8 million people, or 45.3 percent of the population, live in cities of one million people or more -- in Russia, the figure is 33 million or only 12.6 percent of the population. Then there is the lack of discipline. Pipes tells the story of how a Russian walking into a room of 20 people can tell them to leave the city immediately and be listened to, but an American in a similar situation would be laughed at.

The Soviets have an advantage over the Americans in civil defense preparations, Pipes says. "Because here the notion has spread that if there's nuclear war, it's the end of life on earth. That creates a mood of panic, whereas their notion goes to the other extreme. They tell their own citizens: 'Don't worry. It's just like any other war. And you'll survive it if you are calm, obey orders and so on.' "

Comparing anxieties about nuclear war and cancer, Pipes says he knows a man who ruined his life convinced he was going to die of cancer. "He never got it," Pipes says with a grimace. "Yet his whole life was under the spell of cancer."

Pipes draws another parallel: For years people didn't dare mention the word and some people went to their graves with untreated cancer because "we didn't acknowledge it, wouldn't diagnose it and it wouldn't be treated. Now, the fact is that close to half of cancer cases are being cured; people are not ashamed or embarrassed to say they have cancer. The same thing is true for nuclear weapons which ational. Thre a kind of international cancer, which can destroy humanity, were they to get out of control."

Pipes says that the West European disarmament movement, which gathered strength last year, has been on the decline since the imposition of martial law in Poland. "I would be in favor of it if you also had mass antinuclear demonstrations in Moscow and Kiev and Prague," he says. "It would be a great thing. But of course it doesn't occur there so that this is, in effect, a unilateral disarmament movement and what it does is undermine our ability to stand up to Soviet threats. I think people who engage in this are very misguided." He sees all disarmament movements in the West as having the effect of pressure for unilateral disarmament because they lean on only Western governments and have no leverage with the communists.

Pipes cites a newspaper report about women over 30 experiencing great difficulty in conceiving and many of them outraged, feeling "betrayed" by their bodies."Who are they outraged at?" Pipes asks. "Their own bodies? This kind of reaction makes you wonder about human ability to deal with reality.

"Nobody wants nuclear war. The question is not do you want nuclear war or don't want nuclear war, but how do you prevent it. Now if you tell me that the only way to prevent it is by surrendering and becoming an inmate of Gulag, I say no. I not only don't want that, but I don't think this is the true alternative. We can be both free and alive, and if you understand what the threat is and take proper measures, you can survive and don't have to succumb.

"The fear of nuclear war is just as great on the other side as it is on this side. If we have a credible retaliatory power, a balance of terror, they will not strike as we would not have struck Hiroshima if the Japanese had had a nuclear weapon. But the notion that we must have unilateral disarmament does not appeal to me and I am convinced does not appeal to the overwhelming majority of Americans. There is no logic in it."

Pipes does not advocate a big increase in our civil defense preparedness; he doesn't think the American public would buy it. Nor has he ever discussed bomb shelters or evacuation plans with his family. "I must say that neither in my family, nor among my friends and acquaintances, nor among my Harvard students, do I find an obsessive preoccupation with nuclear war. Perhaps it is internalized, but I find the people I know and observe go about their life's business as they have always done."

Pipes says he is much more worried about his children driving safely, and not getting sick, than nuclear war. He has never thought of building a bomb shelter.

Pipes says he does not dream about war, nor does he spend much time thinking about how it might occur.

The Soviet strategic forces are on a low rate of alert, he says. A Soviet strategic strike out of the blue is an impossibility.

Pipes is a charmer, a raconteur; his conversation is studded with historical analogies. He is at his most persuasive when contrasting a Russia steeled in war and misery with an America wanting to believe in a friendly world of comfort and happiness.

He seems a stranger to doubt and worry. He is proud that his family made the right decision in escaping from Poland after a month of German occupation. They took some risks in traveling through Germany, but their luck held: their forged identity papers were not checked out, and eventually they reached the United States.

World War II had a profound effect on his thinking, he says. He learned to expect the unexpected, such as normal life turning upside down in a matter of hours, and he learned to take seriously threats, such as the Nazi pledge to kill all Jews, even assimilated Jews such as his family and his wife's. "The only reason we have survived, both of us," Pipes says, "is because our families didn't lose their heads, but fought back for their lives and freedom. And that's the only way ever to manage your life. The nuclear threat does not change that."

Pipes emphasizes his point by tapping on the table. I think of the thud of the shell that landed on the floor of his bedroom but did not explode.