GEORGE KENNAN has a way of wandering into the room, so to speak, at about that moment when the discussion at hand has reached a critical point, when the issue has been clearly joined, and when a firm voice and a few forceful words can have a powerful impact. He did just that almost precisely 30 years ago in a famous article in Foreign Affairs magazine, written under the readily penetrable pseudonym X, that gave the word "containment" to the language of diplomacy. Kennan would be the first to question whether the influence he is credited with having had on the American conduct of the Cold War was precisely the one he had in mind. (In his later Memoirs, he confesses to "egregious" omissions and ambiguities in the X article, and blames only himself.) But nobody doubts that his prescription for dealing with the emerging Soviet challenge of that time - however much it may have been misread - had a profound effect on policy.

Now he has given us in his latest book a much more comprehensive prescription for the practice of American foreign policy. And while it might be presumptuous to predict the same sort of impact and excitement again - Kennan is writing openly from retirement rather than pseudonymously from within the policy-making councils - it does seem safe to say that he is once more joining a developing debate over American foreign policy at a critical time, and with the same firm voice, compelling logic, and plain purpose of influencing policy.

The question is: Which way? And the answer is that while Kennan offers more reinforcement to the thinking of the Carter Administration than, let us say, the Committee on the Present Danger, he belongs to no camp, perhaps because his extraordinarily firm grasp of the internal workings of the Soviet system is matched by his lamentably loose grip on the way politics and a free press are supposed to work in his own country; the question of what policy might, or might not, be politically practical simply never comes up.

Legend places Kennan in the forefront of the cold warriors - rightly or wrongly, "containment," as a counter-strategy to the encroachments of international Communism, has always had a hard, inflexible, unyielding ring to it. And yet, Ronald Reagan, Barry Goldwater, assorted admirals and generals and kindred spirits who warn of Soviety treachery, push for accelerated American arms and shy away from the word "detente," will find in The Cloud of Danger small comfort and slight support. The "cloud" Kennan has in mind - the central concern which dictates the rest of what he calls his "grand design" - is "presented for all of us by the enormous cultivation and spread of nuclear weapons." Consequently, he says:

"Let us teach ourselves to look at the Soviet problem as a serious political one which has, indeed, military implications, but to bear in mind that these implications are of a secondary, not primary nature . . . The greatest danger inherent in the existing competition between the Soviet Union and the United States in the military field, is not the danger of a Soviet attack on ourselves or on NATO; it is the danger that the momentum of this tremendous and infinitely dangerous weapons race will get out of hand, will become wholly uncontrollable, and will, either through proliferation or by accident, carry us all to destruction."

From this flows an urgent appeal for arms control. It is Kennan's view that "both sides could afford to give up four-fifths" of the nuclear weapons in their possession "tomorrow" and still have enough "to serve all useful purposes." And he adds: "A unilateral reduction of 10 percent, immediately and as an act of good faith, could hurt neither of them."

A spontaneous, unreciprocal, 10 percent reduction of American nuclear capability as an earnest of good faith? Not even Jimmy Carter, with his Kennanesque warning against an "inordinate" fear of Communism, is ready for that. And that is not the only recommendation unlikely to warm the hearts of the White House. Kennan, for example, would press the case for human rights in Russia only up to a point; we should "avoid actions and gestures that smacked of direct interference in Soviet internal affairs." As for the so-called Third World in general, he would begin by taking account "of our general helplessness in the face of its problems . . . because of the necessity of concentrating our resources and efforts elsewhere."

But if the White House may be disappointed by some aspects of the Kennan Grand Design, admirers of the old "containment" as it was originally perceived - or misconceived - may well feel abandoned or betrayed. Kennan advocates "the reduction of external commitments to the indispensable minimum. And I would see this minimum in the preservation of the political independence and military security of Western Europe, of Japan, and - with the single reservation that it should not involve the dispatch and commitment of American armed forces - of Israel." To concentrate on these "essential tasks," Kennan would "ruthlessly eliminate ulterior commitments and involvements." He would perhaps beef up the American contribution to NATO. But he would abandon "obsolescent" positions in Panama, in the Phillipines, and in Korea. He would "tread warily" with China. In the Near East, he would "moderate our enthusiasm for putting fantastic quantities of arms in the hands of peoples unable to make any proper use of them." He would try to "moderate passions" in Southern Africa without "taking sides."

This is "containment"? Well, it's not quite the way he expressed it 30 years ago - a policy "designed to confront the Russians with unalterable counterforce at every point where they show signs of encroaching upon the interests of a peaceful and stable world." But in fairness, it's not all that different from what he later said he meant and from what he was also saying at the time; the "counter-force" he had in mind, he later insisted, was to be "political," not "military," and highly selective in its application. And today's world, of course, is different - and not only because of the nuclear danger. Russian capabilities and limitations can be measured with greater certainty from the experience of the past 30 years. And American intervention abroad, he concedes, is severely circumscribed by the Vietnam experience, by a consequent congressional insistence on a stronger hand in the policy-making process, and by public (and press) demands for a lot less secrecy and a lot more accountability.

Now a Grand Design, in only 234 pages, is a tall order. But, then, X condensed into only 17 pages both "The Sources of Soviet Conduct" (the title of the Foreign Affairs piece) and the appropriate American counter-conduct - which may be where he went wrong. His meaning is less likely, I think, to be misconstrued this time around; he marshalls his arguments with a fine sense of order and authority. He sounds, in short, like a man who has spent a lot of time thinking about his country's proper role in the world and who knows well what he thinks. So it scarcely matters whether what George Kennan is saying about the fundamentals of American foreign policy in 1977 is consistent with what he was saying, or insists that he was saying - or was thought to have been saying - 30 years ago. Whatever the case, I will be surprised if The Cloud of Danger does not create a considerable stir. This is not a memoir or a slice of history of the sort that Kennan has done with distinction over the years. It is, in fact, a grand design. And both the intelligence and integrity of its author and the sweep and cogency of its argument will make it difficult to ignore.