An undisclosed decision by the South African government to break off its dialogue on reforms with the western powers and instead face up to sanctions turned a rare special convention of the ruling National Party this week into a superfluous exercise, according to political analysts here.

When President Pieter W. Botha announced the special convention last January amid a blaze of publicity emphasizing his commitment to reform, it was billed as a watershed event at which delegates would endorse constitutional amendments giving the black majority a role in central government.

But when the 1,700 party delegates assembled in the Indian Ocean city of Durban Tuesday night, there were no amendments for them to consider. Botha instead used the convention as a platform for defiance of sanctions and for drawing more clearly the line beyond which his reform program is not to go.

Even so, as the delegates filed out of the convention hall Wednesday into the balmy subtropical night, after 12 hours of heavy rhetoric, they were wondering aloud why it had been necessary to summon a convention to do that.

What happened, according to some informed political analysts, is that between the announcement and the holding of the convention, the government decided on a change of strategy that nullified the convention's original purpose.

They say the government decided to abandon efforts that had been aimed at assuaging western critics and persuading them, in Botha's words, that Pretoria acknowledged apartheid to be "outdated" and was committed to moving away from it.

The new priorities were to take a public stand against what was perceived as foreign interference and make a determined effort to crush continuing resistance in black townships.

"The whole political climate and context has changed," said Andre du Toit, professor of political philosophy at the Afrikaans University of Stellenbosch, which is considered close to government thinking.

"There has been some backtracking on quite a few reform issues and a clear decision to back out of the whole framework of discussion with the Americans and other westerners that the government was engaged in at the beginning of the year," du Toit said.

He pinpointed the moment of decision as mid-May, when the government was engaged in discussions with a special Eminent Persons Group appointed by the Commonwealth to assess whether South Africa's claims to be dismantling apartheid were meaningful. The 49-nation alliance was seeking guidance on whether to impose sanctions.

Du Toit said those discussions convinced the Botha administration that it could never end the pressure through dialogue and concession. Each policy concession would be followed by a new demand, again backed by the threat of sanctions.

"I think the Eminent Persons Group brought home to them that although they could play it out, there was no end to the process," du Toit said. "They felt they were being pressured into negotiating themselves out of power, so they decided to call a halt and face the threat of sanctions now."

There was a sudden growth of Afrikaner far-right organizations at that time, and on May 22, as the Commonwealth group arrived in Cape Town for another round of talks, the neo-Nazi Afrikaner Resistance Movement rattled the government by breaking up a National Party rally in the northern city of Pietersburg.

Du Toit said this gave impetus to the decision. The right-wing parties were accusing the government of being too compliant in its relations with foreign countries, and a major opinion poll published at the time showed that a growing number of the Dutch-descended Afrikaners, who are the backbone of the government's support, shared this view.

Another charge, which seemed to unnerve Botha, was that the government was failing to end the persistent violence in black areas because it was hesitant to get really tough with the blacks for fear of upsetting the Americans and other outsiders.

Another leading Afrikaner political scientist, Hermann Giliomee, said key decisions were taken in mid-May not to release the imprisoned leader of the African National Congress, Nelson Mandela, nor to lift the ban outlawing the ANC, as the West was demanding.

Such decisions taken in the inner councils of South Africa's secretive government are seldom disclosed, but Giliomee said he has been told that the decision was made by Botha, who felt that releasing Mandela -- who has acquired a Messianic image in the black community during his 24 years of incarceration -- would be too risky.

Once that was decided, other decisions flowed from it, Giliomee said, because Mandela's release and the legalization of the ANC had become the focal point of western pressure.

Frederik van Zyl Slabbert, former opposition leader in the white-dominated Parliament who has been trying to garner support for the idea of negotiating with the ANC, said a significant segment of Botha's Cabinet had come around to his view by mid-May, but that the president ruled it out.

He suggested Botha was swayed by the security chiefs, Defense Minister Magnus Malan and Law and Order Minister Louis le Grange.

Events bear this out. After an initial visit to South Africa last February, the Commonwealth group drafted a plan aimed at opening the way for the first direct negotiations between Pretoria and the ANC.

The plan, which the State Department had indicated offered hope of a breakthrough, involved Pretoria agreeing to release Mandela and to legalize the ANC, in return for its declaring a truce in its guerrilla struggle to overthrow apartheid and agreeing to negotiate.

Early in May, Foreign Minister R. F. (Pik) Botha, who is thought to be one of those who favors Mandela's release, sent a special envoy, Carl von Hirschberg, to London, reportedly to tell the Commonwealth group that South Africa was not opposed in principle to freeing the black nationalist leader and legalizing the ANC but wanted assurances of western backing if it had to act against resulting violence.

Apparently encouraged, the group returned to South Africa to meet Pik Botha on May 13. As some recounted afterward, he arrived late, apparently delayed by a long Cabinet meeting that presumably was making its decision on Mandela. He seemed distressed and accused the group of causing difficulties for South Africa.

President Botha did not see the Commonwealth members. Instead they were advised to attend a speech, in which the president slammed the "unsolicited interference" of "meddling groups visiting the country."

Four days later, after a brief visit to the ANC's exile headquarters in Lusaka, Zambia, and just 30 minutes before they were due to meet with key members of Botha's Cabinet in Cape Town, the group was told that South Africa had just launched a series of commando raids on ANC facilities in Zambia, Zimbabwe and Botswana.

The mission in ruins, the group returned to London that night and prepared a negative report that led to the Commonwealth accepting a sanctions package and that helped influence yesterday's 84-14 vote in favor of expanded sanctions in the U.S. Senate.

Soon afterward, the government's information bureau issued a heavily publicized booklet labeling the ANC and Mandela as communists and making it clear that there was no chance Pretoria would agree to negotiate with them.

Two weeks later, President Botha declared a general state of emergency and his security forces began rounding up an estimated 10,000 political detainees.

Declaring that South Africa was prepared to "go it alone," the president said: "South Africans will not allow themselves to be humiliated in order to prevent sanctions. If we have to be dependent on our own Creator and our own ability alone, then I say let it be."

There remained one more event to underscore the South African decision. Although the writing was on the wall, British Prime Minister Margaret Thatcher, in a bid to deflect pressure for sanctions from her Commonwealth and European Community partners, sent Foreign Secretary Geoffrey Howe to South Africa for a final attempt at persuasion.

The mild-mannered Howe was given a public brushoff. Botha called a press conference to accuse Howe of trying to strong-arm South Africa into releasing Mandela unconditionally and legalizing the ANC.

Then came this week's special convention of the National Party. "Those who want to force us into negotiation with radical elements, by demanding unacceptable concessions through blackmail and a manipulated world opinion, should not underestimate us . . . . I'm not a jellyfish, we're not a nation of jellyfish," Botha declared.