Why the Russians can fish in southern Africa's troubled waters

The peace of southern Africa is today threatened more by the unchecked exercise of South African military might than by Soviet machinations or the presence there of Cuban troops. Given South Africa's anxiety about a Soviet-sponsored ''total onslaught'' directed against white rule, its own attempts to unsettle the region are as doubtful as they are dangerous.

From a tactical perspective, the South African armed forces have been most open and most successful in Angola. In order to limit the field ability of the guerrillas of the South West Africa People's Organization (SWAPO), Namibia's prime liberation group, South Africa has systematically destroyed SWAPO bases in southern Angola, eliminated their effective radar cover, and pushed their command structure well north of the Angolan-Namibian border.

South Africa has demonstrably limited SWAPO's ability to strike into Namibia. It has also underscored the weakness of the government of Angola, and of the Cuban soldiers and East German and Soviet advisers in Angola.

South Africa now controls, on a daily basis, a crucial salient of south-central Angola. There the writ of the Angolan government cannot run. Nor does it run in southeast Angola, or in the rural areas of much of central Angola. The Union for the Total Independence of Angola (UNITA), an antigovernment insurgency force backed by South Africa, holds sway in those sections.

Although Namibia is controlled by South Africa, it is not South African soil. Yet the South African Defense Force has made the effective defense of Namibia, including cross-border raids and the dominance of southern Angola, an integral part of the protection of the South African realm. It would also welcome the replacement of the present Angolan government by UNITA, with the exodus of Cuban troops an obvious prerequisite.

The understandable goal is, on the Israeli model, to provide a buffer zone around South Africa, thus cushioning its isolation and minimizing the threat of border conflicts. If South Africa achieves a cease-fire in negotiations this month with Angola, that will help, even if a halt in hostilities leads to no overall Namibian settlement.

On its other exposed flanks, South Africa is attempting to create a buffer zone based on fear of South African retribution. The overt aim is strategic: to diminish attacks on and in South Africa by Soviet-supplied African National Congress (ANC) guerrillas operating from nearby Mozambique, Lesotho, and Swa-ziland. An assault last year on little Lesotho had that effect. So did an earlier raid on Mozambique.

But raids on independent neighbors form only a part of the South African arsenal. By supporting the Mozambique Resistance Movement (MRM), an antigovernment guerrilla force now operating throughout the old Portuguese colony, South Africa creates a further insecurity, flexes its muscles by proxy, and undermines the credibility of a regime which is friendly with Moscow.

The South Africans may even hope to use the MRM to topple the present government of Mozambique. In a slightly different sphere, but presumably with some of the same objectives in mind, they have already encouraged the MRM to cut oil and gas supplies to landlocked Zimbabwe, where an anti-Soviet but also anti-South Africa government rules.

These repeated actions have hindered the economic prosperity of Zimbabwe and helped contribute to an atmosphere of instability in the new nation. South Africa may also be helping to inflame the wave of banditry which has so demoralized southwestern Zimbabwe since mid-1982.

South Africa's neighbors complain about a pattern of deliberate destabilization. Prime Minister Pieter W. Botha of South Africa has vehemently denied the charge, calling it ''another stunt by Marxist forces.'' But, in the same interview, he also promised to aid fellow Africans ''threatened by the evils of communism.''

For Mr. Botha's government, the Soviet threat is palpable. His critics, however, suggest that by deliberately undermining the stability of the region, South Africa enhances the legitimacy of a Soviet (and Cuban) presence, redoubles the need for Soviet assistance, and intensifies desperate rather than deliberate tendencies in the neighboring states.

If the region were not so threatened, South Africa would have an alternate way of demonstrating and exercising influence. As the region's economic powerhouse, with crucial transportation links, modern industry, and an agricultural sector often in surplus, it now plays and could in future play a dominant role in the modernization of southern Africa. But only within a framework of peaceful coexis-tence.

Much of the region's new instability has developed under the umbrella of the Reagan administration's policy of constructive engagement (being unusually nice to South Africa in order to gain independence for Namibia). Indeed, the fallout of constructive engagement is destabilization. An updated and revised version of constructive engagement could bring American policy choices to bear on South Africa, restore stability to the region, and contribute to a diminution of the Soviet ability to fish in the region's turbulent waters.