What Pretoria hopes to gain from attack

South Africa's unusually deep incursion into Angola means: * Either South Africa has decided it cannot risk independence for Namibia (South-West Africa), and wants to torpedo United Nations and Western power efforts in that direction.

* Or less drastically, South Africa wants to buy more time to try to change the scenario for independence -- to the advantage of its candidate to inherit sovereignty over Namibia, the white-run multiracial Democratic Turnhalle Alliance (DTA), and to the disadvantage of the latter's black nationalist challenger, the South-West African People's Organization (SWAPO).

Namibia, the territory Pretoria runs immediately to the south of Angola, is the last major piece of white-run colonial real estate in Africa -- if one excludes South Africa itself.

SWAPO has long conducted a guerrilla was against South African forces in Namibia from bases across the border in Angola.

Whatever the long-term diplomatic aim of the South African operation in Angola, it is likely to have two immediate effects beneficial to South Africa:

1. Straining, if not splitting the unity of the five-man Western team (United States, Britain, Canada, France, and West Germany) that has been going to and fro in behalf of the UN for more than four years between South Africa and SWAPO to secure an agreed plan on independence for Namibia.

The threatened split is between the US and the four others, basically because , since President Reagan took over in the White House, US policy is perceived as having shifted back into a more sympathetic attitude toward South Africa. Significantly, Britain, France, and West Germany have all publicly condemned the South African incursion into Angola. The Reagan administration has not.

2. Increasing the Angolan government's dependence on the 20,000 Cuban troops that have been in the country ever since independence in 1975. The South Africans probably calculate that the greater the dependence of the Angolans on the Cubans, the greater the sympathy for the South African cause, which can all the more easily be reresented as holding the line against the Soviet Union's proxies in Africa.

The UN blueprint for independence for Namibia has been stalled since the beginning of the year. In oversimplified form, it provides for a cease-fire in the guerrilla war, the concentration of South African and SWAPO forces in agreed bases, and free elections under UN supervision. Most outside observers have been forecasting that those elections would be won by the radical SWAPO and not by the compromise ("stooge" to black nationalists) DTA. That is a prospect hard for South Africa to accept.

In at least one count, the South Africans have prima facie a legitimate complaint. The UN General Assembly long ago recognized SWAPO as the sole legitimate voice of African nationalism in Namibia. So how (the South Africans ask) can any elections under UN supervision ever be completely impartial? There are two other counts on which the South Africans raise questions. What guarantees are there, if SWAPO won the election and its leader, Sam Nujoma, became president or prime minister of the newly independent territory: (1) for Namibia's 110,000 whites, or 11 percent of the total population? (2) for removal of the Cuban presence from neighboring Angola?

Such fears, plus an expectation of greater US sympathy, led South Africa to bring to an abrupt end in January a UN conference in Geneva intended to put final touches to agreement on the proposed independence plan.Since then, the US has been considering how it can amend the draft agreement to make it more palatable to South Africa.

Nothing concrete has yet emerged. US Secretary of State Alexander M. Haig Jr. had been expected to confer on Namibia with the foreign ministers of the four other Western contact powers when they come to New York for the next regular session of the UN General Assembly, due to open Sept. 15. These plans have been overtaken by the furor caused by the South African incursion into Angola, the current security council debate on it, and the wider General Assembly debate on Namibia likely to open Sept. 3.

The outcome of both these UN debates could all too easily lead to frustration and recrimination -- particularly if the Western powers play for time by vetoing any proposed sanctions against South Africa. A measure of black Africa's concern at the present confusion is the dispatch of a contact group from the Organization of African Unity (OAU) to visit the capitals of the five Western ccontact powers for urgent talks on Namibia/Angola. It is due in Washington Aug. 31.

Some third parties close to the Namibia negotiations at the UN say that Washington may still be able to win support from both Europeans and Africans for any amendments it proposes to the Namibia independence blueprint, provided it can guarantee to "deliver" South Africa. But that, of course, is easier said than done.

As for South African Prime Minister Botha, he may see in the confusion at the UN and among the Western powers some consolation for his own growing problems. These include:

* Persistence of -- even an increase in -- hard-line white Afrikaner opposition to any softening of the edges of long-established apartheid policy that he was thought to want to introduce to make it more acceptable and workable.

* An increase in sabotage within South Africa by members of the underground African National Congress emerging now as the republic's authentic black "liberation" group.

* Adverse outside publicity over this month's forced removal of African squatters from the Cape Town area to the black homeland.

* A consequent upsurge in defiance among the black rural homeland population -- at least in the Transkei -- a new phonomenon.