The Carter administration, under prodding from Moscow, in August warned South Africa not to detonate an atomic bomb even though U.S. intelligence had no hard evidence that such a test was planned and strongly suspected it was not.

This remarkable fact casts new light on the peculiar Soviet - U.S. - South African interchange two months ago. Since no detonation was likely and the South Africans need no such test to develop their nuclear capability, the incident really had little to do with the Pretoria government's forcing its way into the nuclear club.

It had everything to do with detente. With U.S.-Soviet relations then chilly, Carter policymakers seized at a chance for cooperation between the two superpowers against a pariah nation. The realities of detente are revealed by the Kremlin's response: a Soviet propaganda campaign that claims full credit for stopping the South African blast and that links Washington with Pretoria.

The entire affair was Soviet-instigated. On Aug. 6, President Leonid Brezhnev cabled President Carter, warning that the South Africans were about to set off a nuclear explosion in the Kalahari desert. In light of later implicit U.S. corroboration of Brezhnev's information, it is highly instructive to report U.S. intelligence's actual view of two months ago:

U.S. satellite reconnaissance had spotted something going on in the Kalahari desert. But whether it was a prospective atomic blast, a missile test or something else was beyond the competence of photo interpreters. Lacking other intelligence, the experts could only guess, and the guess of some was that no explosion was being prepared.

An atomic test grants noisy admission to the nuclear club, but it is not the only or even the most significant entrance ticket. India has detonated an atomic bomb but is less a true member of the club that Israel, which has exploded none but has some 16 bombs in its arsenal.

Test or not, the South Africans are either near or at the point of building a bomb - possibly helped by their friends the Iraelis. What's more, nobody doubts that the white Pretoria regime would use nuclear force if in danger of annihilation by black Africa.

Nevertheless, Brezhnev's cable was a ray of sunshine to U.S. policymakers, then worried about two weeks of Russian silence following President Carter's conciliatory foreign-policy speech of July 21. They felt cooperation on the South African matter might rekindle what in early August seemed the dying embers fo detente (besides furthering the Carter administration's courtship of black Africa).

On Aug. 15, the President answered Brezhnev, noting that satellite reconnaissance showed something afoot in the Kalahari desert. A stiff U.S. note demanding that South Africa halt plans for any test was presented in Pretoria to Foreign Minister R. R. (Pik) Botha by U.S. Ambassador William Bowdler.

"I imagine the Japanese, when they surrendered, were treated with more respect than you're treating me with," Botha told Bowdler. This marked further deterioration in U.S. - South African relations - deterioration that moderates in Pretoria feel undermines racial liberalization there.

Soon after Carter announced there would be no test blast, detailed accounts appeared in The Washington Post and The New York Times of how the two superpowers has collaborated to head off the racist regime. U.S. foreign-policy officials told reporters of improved U.S.-Soviet relations that resulted.

The voice of the Kremlin was not nearly so sweet. The closely controlled Soviet propaganda machine, aiming at black Africa, took sole credit for preventing the test explosion and then linked the West with South Africa. For instance, a Pravda correspondent contended over Soviet radio Aug. 23 that "secret assistance of the Western powers" in South African nuclear development is "an integral part of the efforts of the NATO powers to create an immense military machine in South Africa."

Last month's edition of the authoritative Soviet World Outlook, published by the University of Miami's Center for Advanced International Studies, concluded: "Ignoring American views that U.S. actions in response to Soviet warnings had improved U.S.-U.S.S.R. relations, Moscow instead has been skeptical of Western demarches to South Africa and played up Western ties with that country."

Indeed, the present improvement in U.S.-Soviet relations stems from U.S. concessions on arms control, not developments in Africa. Nor is it even clear that the U.S. intervention averted a test explosion, much less intimidated Pretonria from eventually going nuclear. Last August's reports of a major diplomatic triumph now seem greatly exaggerated.