In the midafternoon on Saturday, Aug. 6, the acting chief of the Soviet embassy, Vladillen M. Vasev, called at the White House with an urgent personal message from Leonid I. Brezhnev to Jimmy Carter. South Africa, according to Soviet intelligence, was secretly preparing to detonate an atomic explosion in its Kalahari Desert. Brezhnev asked for Carter's help to stop it.

The Brezhnev message, still in its original Russian, enclosed the text of an announcement scheduled to be made public two days later by Tass, the Soviet news agency, reporting South Africa's preparations. There was no reference in either document to the Russian spy-in-the-sky satellite photographs that played a key role in Moscow's alarm.

The Soviet diplomat told William G. Hyland, the senior Security Council officer on duty on that quiet afternoon, that Brezhev planned to send similiar appeals for action to the leaders of Britain, France and West Germany. In the Soviet view, failure to head off a South African atomic explosition "would have the most serious and far-reaching aftermaths for international peace and security."

In the two weeks that followed, the United States, the Soviet Union and the three European governments engaged in an extraordinary collaboration intended to spare mankind from another fateful step toward the spread of atomic weapons.

Last Tuesday, 17 days receipt of the Soviet note, Carter was able to announce at his press conference that South Africa had promised that "no nuclear explosive test will be taken . . . now or in the future."

For the first time since the dawn of the nuclear age in a blinding flash over Hiroshima on Aug. 26, 1945 - 32 years to the day from Brehnev's message - the world's leading powers, east and west, worked in concert to back away a lesser nation from the threshold of entry into the nuclear weapon club. If this cooperaion can be buttressed and extended - and if timely warning through intelligence is available in the future - what happened without much public notice in these past weeks may set a pattern of historic importance.

Despite South Africa's denials that any nuclear test was ever planned, imformed U.S. officials believe the evidence to the contrary is overwhelming. Clear and detailed U.S. reconnaissance satellite photographs, ordered by the White House within hours of the Brezhev note, showed construction in the desert that experts said was typical of a nuclear test site. In the absence of the outside pressures, administration experts said, South Africa might have gone on to detonate a bomb there within a matter of weeks, assuming that the explosive material was in hand and that it chose to move full speed ahead.

Some American intelligence specialists, dissenting from the administration consensus, suspect that the construction in the Kalahari Desert was an elaborate sham intended not for an explosion but for the shock value interpretation, white South Africa intended to dramatize at minimal cost its claim to be a nation of major capabilities which will not permit outsiders' versions of racial equality to be shoved down its throat.

The hoax theory is heavily discounted inside the Carter administration, which believes that the Kalahari preparations were real. In view of the nuclear sophistication of South Africa, the Carter administration concluded that what was being built could be used. All U.S. planning proceeded on that premise.

If South Africa had set off an atomic device, the shock waves could have reverberated with even greater force than India's May 1, 1974, explosion which shook the major nuclear supplier nations into belated cooperative efforts to stop the spread of dangerous materials and technology. This drive to head off a nuclear-armed world, centered on the periodic semi-secret mettings in London, would have been gravely set back and its future cast in doubt - especially because South Africa possesses abundant depositsof natural uranium. If it could process and enrich enough uranium to make its own bomb, it potentially could export weapons-grade material to other counteries.

In African terms, Pretoria's detonation of a bomb would have been a signal of defiance with profound impact on the American-British effort to forestall widespread racial warfare in southern Africa. And it would have been a stunning rebuff to President Carter's policy of direct, explicit pressure on South Africa to abandon its internal racial policy of apartheid.

A United Nations-sponsored world conference against apartfeid was due to convene Aug. 22 in Lagos, Nigeria, with many black African leaders to be in attendance. This fact and the intense Soviet competition with the West for influence in Africa raised initial suspicion in Washington and European capitals that Brezhev's warning was simply a propagada exercise.

Beginning with the Tass announcement of South Africa's nuclear preparations and continuing through two weeks of uncertainty over the outcome, the Soviets publicly blamed the West for atomic collaboration with South African "racists" even while privately consulting the West on ways chotomy was characteristic of the two to block a nuclear explosion. The difaces of today's superpower detene, where bitter rivalry coexists with wary cooperation in pursuit of shared objectives.

Hyland, the acting National Security Council director on duty in the vacation-depleted White House on Aug. 6, is a veteran of two decades of U.S.-Soviet diolomacy and knows the Russian language. As Soviet embassy minister Vasev gave an oral summary of message. Hyland took notes and, when the diplomat handed over the paper, quickly reviewed the Russian text.

Hyland told Vasev he would convey the message promptly to the President who was on vacation in Plains. Georgia, and that the United States, after making its own study, would respond to Brezhev.

As soon as the envoy departed, Hyland telephoned his boss, presidential national security affairs adviser Zbigniew Brzezinski, who was on vacation in Maine, to coordinate the alerting process. Hyland next telephoned Plains. He relayed the substance of Brezhev's message to White House press secretary Jody Powell, who had just returned from a presidential news conference on the previous preoccupying issue of the day. Carter's proposed overhaul of the nation's welfare system.

Telephone calls then went in quick succession from Hyland to Deputy Secretary of State Warren Christopher, acting in the absence of Secretary Cyrus R. Vance Vance was in Amman, Jordan, midway through grinding search for a diplomatic bridge across the Arab-Israeli divide.

The next Hyland call was to Adm. Stansfield Turner, director of Central Intelligence Agency and overall chief of American intelligence. An intial assessment was ordered for Monday, Aug. 8, with further reports as vailable.

South Africa's nuclear weapons potential has concerned world leaders and their intelligence services for several years. With its rich uranium deposits as a base. South Africa has over time acquired U.S. technological training, a research reactor and highly enriched fuel to operatet it. West Germany enrichment technology, and signed contracts for two large French nuclear power-generating reactors. It was clearly on its way to building a major independent nuclear industry.

Last February in a comprehensive report that attracted the attention of officials, Washington Post correspondent Jim Hoagland reported from Pretoria that South Africa was within two to four years "at the outside" of being able to manufacture an atomic bomb. Hoagland quoted a U.S. government source as saying that the delay could be reduced to " a matter of months" should South Africa mount a crash program. The next day French Prime Minister Raymond Barre said publicly that South Africa "already has nuclear military capablity."

Despite the warning signals, U.S. intelligence was behind the Russians in picking up the activity in the Kalahari Desert near South Africa's border with its disputed territory of South-West Africa (Namibia). As a consquence of Brezhev's message, a U.S. reconnassiance satellite with high-resolution cameras was urgently programmed for low-orbit passes over the area in question.

Administration officials refused to specify whether high-flying SR-71 reconnaissance aircraft or other means were also in the Kalahari Desert surveillance.

Officials who have seen the results said a cluster of sheds and other buildings around a prominent tower, along with a solidly built structure a bit removed from the rest, can be clearly seen in remote and desolate stretch of sand. To the untutored eye their purpose is nuclear, but to the eye of experts the pattern of preparations around an instrumentation tower is familiar.

"I'd say we were 99 percent certain" that the construction for an atomic test, said one U.S. official. Another official remarked "people were pretty confident that this was what it might be." Another said "we were not 100 percent sure but technicians were less iffy than they had been about some incidents in the past." Still another official said: "It was very likely [a nuclear test site] but with some ambiguity about the purpose."

While the U.S. inquiry was taking place, the Soviet Union was pursing the diplomatic path which Brezhev had forescast. The Russian initiatives showed an interesting variation in timing, amounting to a pecking order on the influence the major powers could exert in the Kremlin's judgment.

The message to Carter on Aug. 6 was followed by another delivered to French President Valery Giscard d'Estaing on Aug. 7. A third was delivered to British Prime Minister James Callaghan on Aug. 8. The following day, Aug. 9, the Soviet ambassador to Bonn deliveres Brezhev's message to West German Foreign Minister Hansdietrich Genscher.

Precisely as Brezhev had stated, the Tass statement clattered across its news wires on the evening of Aug. 8, Moscow time, declaring that according to imformation reaching here, work is presently nearing completion in the South African Republic for the creation of the nuclear weapon and preparations are being held for carrying out its test." The statement said South Africa bases its military strength on help from "some Western states incorporated into NATO as well as on Israel. "And it pointed out the South Africa is one of the nations that has refused to sign the Nuclear Non Proliferation Treaty which would require it to forswear the production of atomic weapons.

With the European capitals notified and the Tass announcement public, diplomatic soundings intendified between Washington and the allied powers. And early decision was that the United States, Britain, France and West Germany would act in coordinated and parallel fashion but would not seek to present jointly agreed statement or take joint actions because of the time-consuming clearances this would involve. The Soviet Union, which has no diplomatic relations with South Africa and no influence with its government, would be kept generally informed of the allied enterprise.

By Wednesday, Aug. 10, the four allied powers had all taken premliminary diplomatic soundings in Pretoria on South Africa's reponse to the Soviet accusation. These probings extend over the next 10 days as Pretoria publicly issue indignant denials of the Soviet charge and privately sought to dispel growing Western belief that the Soviet alarm had merit.

"The first response from South Africa certainly didn't ease our concern," said a U.S. source. Another administration official said, "It only raised our apprehensions."

President Carter returned form Plains that Wednesday night, primarily to be in Washington to welcome negotiators Ellsworth Bunker and Sol M. Linowitz returning the next day from Panama with agreement on the new Canal Zone treaties. But what was being pieced together about the activity in the South African desert was much on his mind.

By Thursday, Aug. 11, the basic assessments, intelligence and diplomatic, were ready for the President. "By then we were able to lay out the problem pretty well for the President," said a senior administration official. Carter wanted to talk to Vance and Brzezinski before taking definitive action.

The problem was intricate: to disuade South Africa from a nuclear explosion; to respond to Brezhev on a clear to prevent Soviet charges of Western collaboration with an atom-armed South Africa from dominating the coming Lagos conference on apartheid and polarizing Africa's black leaders against the West. At the same time, Carter wanted to avoid driving South Africa deeply into a siege mentality that could produce irrational leverage which Pretoria holds for aiding a peaceful settlement of the racial crises in neighboring Rhodesia and in Namibia.

Vance, at this point on Aug. 11, was arriving in London for two days of talks on southern Africa at the end of his Middle Eastern journey. The Secretary of State saw South African Foreign Minister Roelof F. Botha during the London stopover, which was centered on the latest Anglo-American plan for racial transition in Rhodesia, but did not feel ready yet to control Botha with the nuclear question. Before leaving London, however, Vance privately advised British Prime Minister Callaghan and Foreign Secretary David Owen that the emerging U.S. assessment matched that of the Soviet Union.

Vance returned to Washington on Saturady night, Aug. 13, and went to the White House the following day for the meeting Carter had scheduled in his absence. Ostensibly the Carter - Vance - Brezinski meeting that Sunday afternoon was to hear Vance's report on his talk in the Middle East and London. Actually it was also the decisive strategy session on the South African nuclear issue.

The President ordered a full reply to Brezhev sent off the next day, Monday, Aug. 15. In essence it confirmed that the American assessment parallelled his: that there was sufficient evidence to suspect south Africa of preparing a nuclear test which would have international consequences.

With that message the Carter administration was committed to act on the conclusion that South Africa, for global reasons, must be forestalled. But Carter was also determined to preserve his ambitious African policy of mediation. That meant perserving a relationship with South Africa even while exerting pressures in the nuclear arena.

France held a key to the problem because a French consortium contracted in 1976 to sell South Africa two 922-megawatt nuclear power reactors, the ceterpiece of a program to provide 10 percent of the country's power by atomic means within a decade. The United States had agreed in principle to supply the necessary low-enriched uranium to operate the French reactors until South Africa could expand its own pilot enrichment facilities to commerical scale in the mid-1980s.

U.S. ambassador-at-large Gerard Smith, a veteran of nuclear diplomacy, was summoned from vacation and sent to Paris on Aug. 17 to see French President Giscard d'Estaing. Smith took with him the intelligence data accumulated by the United States. It is not known whether, as in the case of Dean Acheson and Charles de Gaulle during the Cuban missile crisis of 1962. Smith spread before Giscard d'Estaing the photographic evident of Washington's conclusions. But it was the data brought by Smith that French Foreign Minister Louis de Guiringaud publicly cited on Aug. 22 as " the more precise indications" France had received that South Africa was preparing a nuclear explosion.

Shortly after the Smith-Giscard meeting in Paris the really tough bargaining with Pretoria began, according to U.S. officials. They would not specify what countereractions might have followed a South African decision to proceed. French sources made it plain to reporters that there might be no limit on the consquences on its part, from termination of the nuclear reactor contrasts to a full break in diplomatic and trade relations. The United Stated at a minimum, would have been bound by Carter administration policy to terminate its nuclear fuel arrangements after an explosion. And Pretoria was well aware that only the United States and its European allies stand in the way of the total economic embargo against South Africa demanded by many nations.

The negotiations came to a head last weekend when the United States, according to informed sources drafted a clear statement of the assurances it sought and transmitted them to Pretoria. "We were pretty severe in private," said one U.S. source. Last Sunday morning, Carter received word that South Africa had agreed.

At his press conference Tuesday Carter announced the agreement in a nearly word-for-word rendition of the diplomatic exchange.

Carter said, "In response to our own direct inquiry and that of other nations, South Africa has informed us that they do not intend to develop nuclear explosive devises for any purpose, either peaceful or as a weapon, that the Kalahari test site which has been in question is not designed for use nuclear explosives, and that no nuclear explosive test will be taken in South Africa now or in the future."

What are the changes that, despite the assurances, South Africa will decide sooner or later to detonate a nuclear explosion. Nobody can say a nuclear explosion. But it is certain that the consquences woult be even graver now that before the Brezhev alarm and the Multipower diplomacy. Blatantly to break its word, as publicly announced by the U.S. President, would place South Africa almost in the position of an international outlaw.

Despit the announced assurances, American officials warn that the sequences of events is not complete. In welcoming South Africa's statements Carter added Tuesday that "we will, of course, continue to monitor the situation there very closely." And he announced that the United States is asking South Africa to sign the Nuclear Non-Proliferation Treaty and to place all its atomic facilities, including its pilot uranium enrichment platn, under international inspection.

South Africa, during the height of the controversy, ridiculed charges that it was on the verge of an atomic test. Foreign Minister Botha called the initial Soviet accusations "wholly and totally unfounded." When France joined in the accusations last Monday, South African Under Secretary of State Brand Fourie called the French allegation "incomprehensible."

Last Wednesday, the day after the Carter announcement, South African Prime Minister John Vorster sarcastically referred to the "so-called discovery" by the Soviet Union that his nation was about to explode a nuclear device.

South Africa was "kicked out" of the International Atomic Energy Agency (by Soviet agitation, it charges), Vorster wryly noted, last June.

But, Vorster told a Capetown audience, now fewer than 13 of the 34 countries on the board governors of the atomic agency either had not ratified or accepted to the Nuclear Non-Proliferation Treaty.

These nations, Vorster said, are Argentina, Bangladesh, Brazil, Colombia, Chile, Egypt, France - at this point Vorster said he wished to pause and say, "Et tu, Brute" - plus India, Indonesia, Niger, Pakistan, Panama and Portugal.

Why "this blatani discrimination?" Vorster asked, South Africa, he said, "has scrupulously honored its obligations." He noted that the United States, the Soviet Union and Great Britain, while parties to the treaty, protected their own positions by amassing nuclear arsenals.

His nation, Vorster said, was constantly being subjected to "double standards." Said Vorster: "I wishhto say in all sediousness and in all humility that if these things continue . . . the time will arrive when South Africa as small as she is, will have to say: So far, no further. Do your damndest."

The great powers did their damndest between Aug. 6 and Aug. 23. the results, while not yet conclusive, will weigh on the scale of the world's nuclear future.