Johannesburg — Negotiations for the independence of Namibia (South-West Africa) have taken an important step forward with the agreement of all parties on a set of constitutional principles for the territory.
But analysts increasingly look upon such settlement details as a sideshow. The crunch issue has become the withdrawal of Cuban troops from Angola. Achieving that will be ''very difficult,'' according to observers here.
The United States, which is leading the negotiations for the Western ''contact group,'' has stressed from the beginning the need for a Cuban withdrawal as an important ''parallel'' issue. South Africa now wants to link the two. Prime Minister Pieter Botha has made a Cuban exit a precondition to ending South African rule over Namibia.
Why? Pretoria argues that it is necessary for South Africa's own security. If the Cubans, estimated to number between 18,000 and 20,000, remain in Angola, they will influence and perhaps even infiltrate a new government in Namibia, South Africa reasons. Their presence in Angola would also greatly influence the election outcome in favor of the SWAPO (South-West African People's Organization) guerrillas fighting for Namibian independence.
However, many analysts here suggest Pretoria's prime motivation in seeking withdrawal of Cubans is more tied to domestic politics.
South Africa has for years warned of the communist ''onslaught'' in the region, and of how a SWAPO government would further Russian aims in southern Africa.
''Pretoria needs to demonstrate that for the price of a SWAPO government, which is likely, there will be the benefit of the Cubans leaving,'' says one knowledgeable Namibia watcher.
There is also the view here that South Africa has raised the Cuban issue to give it an ''out'' should a Namibia settlement ultimately not shape up to its liking. This implies that Pretoria is still not sure it wants to settle the festering Namibia question.
One informed political analyst says the Cuban issue may well be just further evidence that the government remains ambivalent. ''South African foreign policy with regards to Namibia has been the very picture of ambiguity'' since the republic took what the United Nations considers illegal control of the territory over 30 years ago, he says.
While South Africa has explicitly called for Cuban withdrawal before an election, most experts here feel that was just a ''hard pitch aimed high,'' as one put it.
From the South African point of view there is expected to be more flexibility than is publicly evident with regard to any Cuban departure. Angola has so far insisted the presence of Cubans is a purely domestic concern.
The one area of agreement here is that the costs of the Namibian war to South Africa have never been higher. Prime Minister Botha calls the territory a ''millstone'' around South Africa's neck.
South Africa props up the territory with about $1 billion a year of economic assistance, while its own economy is sinking into a phase of no growth.
The loss of life in the war is rising as well. In the first half of 1982, 594 SWAPO members were killed along with 47 South African security force members, according to Pretoria.
The principles that have been agreed to and submitted to the United Nations set out the broad framework of a future constitution for Namibia. Its final form would have to be approved by two-thirds of the elected assembly.
The debate over the actual voting system - constituent or proportional - that led to a breakdown in negotiations a few months back has not been resolved. But all have agreed not to let it delay a settlement.