S. Africa to 'disengage' from Angola

South Africa is going ahead with its unilateral offer to ''disengage'' its military forces from southern Africa, Prime Minister Pieter W. Botha has announced.

The stated intent is to achieve a lasting cease-fire along Angola's troubled southern border.

But close observers read more into the move. They say it is designed to:

* Counter international criticism of South Africa as a ''destabilizing'' force in the region. South Africa hopes a ''peace offensive'' will allow it to capture the high ground in the border conflict with Angola.

* Provide the United States with at least a small victory for its controversial policy of ''constructive engagement'' with South Africa in the US election year. The Reagan administration has adopted a more empathetic stand toward South Africa's white-minority government than some of its predecessors, partly in hopes that its policy would lead to independence for Namibia (South-West Africa).

A US-led initiative for Namibian independence recently appeared stalled. But a cease-fire in southern Angola would be seen as at least peripheral progress toward a possible Namibian settlement.

South Africa first made the offer to disengage from southern Angola during its recent six-week incursion into that country. The offer was for a one-month trial disengagement by South African forces as long as Angola and South West Africa People's Organization (SWAPO) guerrillas ''would not exploit the resulting situation.'' SWAPO is fighting for control of Namibia from bases in southern Angola.

South Africa has not made precisely clear what it means by a disengagement. It has not stated whether all its troops would be removed from southern Angola or whether some would simply be frozen in place.

Both Angola and SWAPO have publicly insisted that a cease-fire be linked to South African implemention of a United Nations Security Council plan for Namibian independence. But South Africa has refused direct linkage.

It is not clear if either Angola or SWAPO have privately agreed to abide by South Africa's cease-fire terms. Botha said South Africa was implementing a disengagement after ''assurances'' were received by the US.

Botha reiterated that the success of South Africa's disengagement - and ultimately of a cease-fire - would depend on ''all the parties'' concerned.

In the view of many analysts, South Africa has successfully seized the initiative by in effect imposing a cease-fire. Andre du Pisani of the University of South Africa says the initiative is partly an attempt by Pretoria to appear to be the peacemaker in the Namibia dispute and to put the onus on Angola and SWAPO to uphold the disengagement and cease-fire.

A cease-fire would by itself be a major achievement, although its relation to Namibian independence would be indirect.

In the past, no cease-fire proposal has ever been agreed to by antagonists in the Namibian dispute.

The key issue that has stalled Namibian independence is South Africa's demand , backed by the US, that Cuban troops in Angola be withdrawn. Angola says the Cubans are an internal matter with no bearing on Namibia.

Informed observers here expect a cease-fire to be followed by further talks between South Africa and Angola, and possibly for the first time direct negotiations between SWAPO and the South African administrator general of Namibia. Botha hinted that trilateral talks involving the US, South Africa, and Angola could take place.

The Cuban issue is complicated by the Angolan government's security situation. It is being challenged by the rebel National Union for the Total Independence of Angola. South Africa is widely believed to support UNITA.

Some hope South Africa might agree to end alleged support for UNITA and that Angola might then set a timetable for sending Cuban troops home.