For the first time in memory, black South Africans have forced the white segregationist government to back down on a major issue involving the ideology of apartheid or racial separation.

Yielding to a six-month opposition campaign, the government has shelved plans to hand over two tribal regions to the neighboring African kingdom of Swaziland. The proposal was designed in part to strip South African citizenship from blacks identified as belonging to the regions.

The leaders of the two regions went to court four times and won, and the government capitulated with a fifth judgment about to be handed down.

The government has appointed a commission to study the matter, but few observers doubt that the issue either will be allowed to fade away quietly or that a much more modest land offer will be made to Swaziland.

"I am delighted," said black leader Gatsha Buthelezi. "This is a great moral victory for our black alliance."

The importance of the backdown is that the land deal represented a major step in furthering the South African government's policy of social and political segregation.

The policy objective is to turn all black South Africans into statutory foreigners by making them citizens of small tribal "homelands" that are given nominal independence.

As each homeland becomes independent, all members of the tribe officially regarded as associated with it automatically become its citizens, whether they live in the homeland or not.

By this process, the 4 million whites eventually will constitute a legal majority over the 21 million blacks who, being foreigners, will lose all future claim to political rights in the major part of South Africa.

The snag is that some homeland leaders are refusing to accept independence, despite considerable personal inducements. The most important is Buthelezi, the strongly anti-apartheid head of the KwaZulu homeland and chief of 6 million Zulus, South Africa's largest tribe.

Another opponent is Enos Mabuza, head of the Kangwane homeland situated around Swaziland's northern and western borders and officially regarded as the tribal home of South African Swazis.

To get around this obstruction to its plans, South Africa decided to try to give Kangwane and the Ingwavuma region of KwaZulu, which abuts Swaziland's eastern border and extends to the Indian Ocean, to the little landlocked kingdom in settlement of claims that it has been making for several years.

This would have had the double advantage of making another 1 million black South Africans foreigners and of placing the Swaziland government in Pretoria's debt.

Because Swaziland is strategically situated between South Africa and left-leaning Mozambique, which is sympathetic to guerrillas of the black liberationist African National Congress, this also would have created a useful buffer.

But Pretoria failed to reckon with the determination and resourcefulness of Buthelezi and Mabuza. The white government is unaccustomed to blacks putting up any serious resistance to its plans.

When Buthelezi and Mabuza objected, the government ignored them and issued proclamations taking over Ingwavuma, in preparation to giving it to Swaziland, and dissolving Mabuza's administration.

It locked Mabuza and his ministers out of their offices, took away their official cars and threatened to evict them from their houses.

Instead of submitting in the face of this action, as the government expected, both leaders went to court. They also lobbied for their cause, calling protest meetings and sending emissaries abroad to urge Western governments to put pressure on Pretoria and members of the Organization of African Unity to pressure Swaziland.

Buthelezi won three cases nullifying the government's proclamations in the supreme court of Natal Province. When the government took the matter to the appeals court last month, Buthelezi won again.

Mabuza sought nullification of his dismissal in the supreme court of Transvaal Province. The government then sought an out-of-court settlement Sept. 25, offering to reinstate Mabuza and his ministers and to refer the matter to a commission of inquiry.