IN JANUARY 1950 the top rank of the United States government was engaged in a highly secret, intensive effort to make two fateful and related decisions: whether to build a hydrogen bomb a thousand times more powerful than previous atom bombs, and whether seriously to pursue negotiations for the international control of all atomic weapons.

The then counselor of the State Department, George F. Kennan, in a 79-page top secret memorandum to Secretary of State Dean Acheson, argued for taking risks, if necessary, to curb the role and growth of atomic weapons lest they strongly distort military planning and national psychology alike.

Kennan's fear was that atomic weapons remaining in American arsenals "may tend to affect our concept of what it is that we could achieve by the conduct of war against the Soviet Union" and hold out the "vague and highly dangerous promise of 'decisive' results, of people 'signing on dotted lines,' of easy solutions to profound human problems." Though the dangers of international control would be great, he agued, the dangers of lack of control would be even greater.

Only 10 days after the submission of the memorandum, President Truman announced the national effort to develop the hydrogen bomb. Kennan realized then that his argument had been in vain.

Last week, more than 30 years later, Kennan stood at the podium of a Washington hotel to describe a nightmare become reality, speaking in anguished terms of a drift toward disaster as the superpowers' atomic stockpiles grow even higher and their political relations ever more precarious. To stop the momentum of the Soviet-American "collision course," Kennan proposed, the two nations should agree to an immediate, across-the-board slash of 50 percent in their nuclear arsenals.

It was a radical proposal from a figure not known for radicalism, a cry for attention to grave and yet strangely invisible dangers of eventual nuclear holocaust. Accepting the $50,000 Albert Einstein Peace Prize in the memory of the mathematician who made splitting the atom possible and then decried the results, this spare and scholarly figure accepted a charge "to neglect nothing -- no effort, no unpleasantness, no controversy, no sacrifice -- which could conceivably help to preserve us from committing this supreme and final folly."

George Kennan, at 77, has had two distinguished careers: 25 years as a professional diplomat specializing in Soviet-American affairs, rising to the top ranks of the U.S. government, and nearly 30 years as a historian and author, twice winner of the Publitzer Prize and the National Book Award. Last week, Kennan seemed to have crossed the threshold of a third and unaccustomed career: that of political activist.

In an interview preceding his Einstein address, Kennan discussed the twin disorders which have brought him to his present state of high alarm. They are the still growing, even accelerating atomic weapons race and the poisonous political connection and equally perilous disconnection between the United States and Soviet Union.

Neither of these conditions is new to the concern of Kennan. Forever labeled as the author of the post-World War Ii policy of "containment" of the Soviet Union because a long diplomatic telegram from Moscow in 1946 and a highly publicized article in 1947 were its intellectual underpinning, Kennan in fact has long since separated himself from that slogan. His current thinking, therefore, represents a culmination rather than a shift of his positions.

"The older I get the more I see all these problems of our political differences with the Soviet Union and with other countries as having only a relative and impermanent reality," Kennan began. "Thirty or 40 years from now there will be a different question about Poland, there will be different questions about Afghanistan -- all these things change but the one thing that could be really final and could destroy all our values is the use of the weapons of mass destruction."

In 1977, in a book titled, "The Cloud of Danger," Kennan proposed a 10 percent unilateral reduction in U.S. nuclear weapons stocks as "an act of good faith" to encourage serious disarmament on both sides. Even in those early months of the Carter administration, when the new president was speaking fervently of nuclear dangers, no such gesture was seriously considered.

Kennan said he decided on a more drastic proposal, a 50 percent immediate, across-the-board and voluntary reduction by both superpowers, because "I feel a greater sense of urgency and a need to jolt people into the realization of what's going on." He had little confidence in the SALT negotiations to find a way out of nuclear peril, but is dismayed at the seeming destruction of even that slender hope.

The United States, and reportedly the Soviet Union as well, are building their nuclear arsenals at a rapid pace. According to Kennan, who obtained the estimate from scientific friends, the arsenals of the two powers already contain over one million times the total destructive power of the U.S. A-bomb that flattened the city of Hiroshima, Japan, in a blinding flash and mushroom cloud in August 1945.

Even a million Hiroshimas, which Kennan calls "grotesque" overkill, is not enough. According to a report from the Center for Defense Information, about 17,000 additional nuclear weapons are scheduled to be produced by the United States alone in the decade of the 1980s.

This is all the more alarming to Kennan because of the intensification of the "war atmosphere" in Washington and the "militarization of thought and discourse" which he condemned in the last year of the Carter administration, after the Soviet invasion of Afghanistan. Since then the election of Ronald Reagan has raised anti-Sovietism to the unchallenged central objective of American foreign policy, brought the former NATO commander, Gen. Alexander M. Haig Jr., to the post of secretary of state and spurred a military buildup which, in constant dollars, will cost more than twice as much as the Vietnam War. All so far without much protest or dissent.

Military apprehension about the Soviet Union, Kennan told me, "has attained the dimensions of something close to a hysteria. And I think that it is extremely dangerous because it has expressed itself in national policy to the point where we have almost no cushion of communications to fall back on in case of serious conflicts with the Soviet Union."

Kennan said he is "mystified" not only by official pronouncements but by "the whole mood in the official establishment of this country" as well as the press and public opinion. "Mystified" because his experience with the Soviet Union goes back 53 years, longer than perhaps any other person still in public life on either side. While he acknowledges that the U.S.-Soviet military balance has shifted, he can see no cause for panic.

"I know the difficult aspects of the Soviet political personality. I know the problems presented for any other power in dealing with the Soviet Union. I understand as well as anybody those aspects of Soviet treatment of its own population which are unattractive and repulsive to us.

"I was there during the [Stalin] purges. I was there during the Korean War when Soviet-American relations were at a low ebb. I think I understand these things as well as anyone in this country.

"What I am not able to see is that any of these things became appreciably worse suddenly in the early 1970s than they were in earlier years. I see no reason to work ourselves up into a lather of military apprehensions suddenly in the middle and late 1970s. But that is what has happened.

"It was never easy to deal with this government. But it's no harder today than it used to be.

"The Soviet government and previous Russian governments always cultivated military strength, particularly conventional land power strength, in dimensions for which nobody else could see the necessity. They always maintained ground forces, particularly along the western border of Russian, which bewildered and frightened other people. This has been going on for 150 years," Kennan said.

Sitting in the quiet study of his son-in-law on a pleasant, tree-lined street, I asked Kennan to consider two of the aspects of Soviet power which have raised the level of Washington concern.

He agreed that, "Yes, there's something to" the perception of greater Soviet global reach, the ability to send its ships and planes around the world and on occasion support the military efforts of the Cubans or Vietnamese in remote areas.

This certainly represents "a difference in their capabilities" and a shift from ideology to military assistance as the predominant means of Soviet influence-seeking. But this "overseas imperialism," as Kennan labeled it, does not weigh heavily on him "because it does not seem to have been greatly successful," and where it has succeeded the gains often have been fleeting. "Nothing that they have gained comes anywhere near to rivaling the cumulative losses of Yugoslavia, China and Egypt," in his view.

And yes, "I would not challenge at all, so far as the outside civilian can judge these things, the proposition that there has been an extensive shift in the balance of military power between the Soviet Union and ourselves." But this, in his view, is "much more because of our own deficiencies than because of any sudden excess of aggressive intention on the part of the Russians."

The United States began to lag in overall military potential at the time of Vietnam because of the war's effects combined with inflation, oversophistication in weaponry and neglect of military personnel, Kennan said. "Our effectiveness obviously fell off. Theirs continued to grow at the same rate. This seems to me to be more of a reproach to us than to them."

While conceding Soviet gains in reach and resources, Kennan said, "I feel that much of our recent reaction has been subjective, not objective." Perhaps this was "a delayed reaction to our humiliations in the Vietnam War and in the Iranian hostage crisis. We're not used to having such difficulties."

Perhaps the most serious danger of all, he went on to say, is that fear and hysteria in Washington could breed the same in Moscow's fertile ground, with the devastating consequences of a self-fulfilling prophecy. "It seems to me that if they are put into a position where they see no hopes of achieving anything in their relations with us except by preparing for a war . . . they'll begin to rule out the possibilities of achieving anything by discussion with us, and to consider only those options which would affect a military confrontation."

The upshot of this, perhaps in the successor Soviet leadership rather than the present one, could be the abondonment of the Brezhnev commitment to detente with the United States and a more confrontational line in all areas. As a historian, especially one whose most recent research has explored the deadly interaction of European states which led to World War I, the possibility "frightens" Kennan. "Once people begin to accept that a given war is inevitable they behave in ways that make it inevitable whether they were right in their initial assumption or not."

Throughout his career Kennan has been known more for his forceful analysis and powerful words, some of which spurred the United States to contest the Soviet Union, than for the practicality of his recommendations. And so it may be today. Following his speech, acting White House press secretary Larry Speakes had no comment on Kennan's arsenals-cutting proposal, and seemed surprised even to be asked.

In a speech last October in West Germany, Kennan declared, "We are being swept by current which we do not understand and over which we have no command. . . .Not for 30 years has the political tension reached so dangerous a point as it has today. Not in all this time has there been so high a degree of misunderstanding, of suspicion, of bewilderment, of sheer military fear."

His remedy at that time was to suggest "a powerful chorus of voices, from the outside, to say to the Russian and American decision-makers what should be said to them:

"For the love of God, for the love of your children and of the civilization to which you belong, cease this madness. You are mortal men. You are capable of error. You have no right to hold in your hands -- there is no one wise enough and strong enough to hold in his hands -- destructive powers sufficient to put an end to civilized life on a great portion of our planet.

"You have a duty not just to the generation of the present -- you have a duty to civilization's past, which you threaten to render meaningless, and to its future, which you threaten to render nonexistent. No one should wish to have in his hands such powers. Thrust them from you. The risks you might thereby assume are not greater -- could not be greater -- than those which you are now incurring for us all."

It was the Kennan message of 1950, translated from State Department bureaucratese to passionate form. Today as then, there is not much evidence that the leaders are listening.