It was pressure from the Anglo-American power structure that did.
http://allafrica.com/stories/201002160438.html...But the real story of South Africa's giant leap from utter darkness, i.e. obnoxious white minority rule, to true freedom, i.e. black majority rule, may have begun in earnest in the mid-1980s during previously unannounced secret meetings between a litany of political stakeholders, including the leaders of the white minority regime in Pretoria, leaders of the A.N.C. movement in exile and the organization's best-known crusader, Nelson Mandela, who had been languishing in the obscure Robben Island prison, as well as representatives of the United States and British governments.
The man who spearheaded the negotiations on the side of the apartheid government was an ageing, frail-looking Pieter W. Botha, the then president of the republic. Urged on by its staunch allies in Washington, he set the cards on the table before the A.N.C in exile, whose most prominent backers were the erstwhile O.A.U, or Organisation of African Unity; Col. Moammar Qadafi's Libya and the defunct Soviet Union....
London and Washington were equally adamant. They would back the unconditional unbanning of the A.N.C. and the release of Mandela and other anti-apartheid activities, provided the nuclear weapons issue was dealt with once and for all, lest a future A.N.C government provide Libya's Qadafi with technology or actual material.
Those were the precursors to a series of monumental events to take place in South Africa, starting February 2, 1990. The man who set the ball of change rolling those twenty years ago was not P.W. Botha, but, the apartheid regime's educational minister, F.W. De Klerk, who had succeeded the ailing Mr. Botha as state president, months earlier. So, exactly two decades after announcing that Nelson Mandela was to be released from 27 years of imprisonment at Robben Island, Frederick De Klerk, in effect the last white President of South Africa came out on Tuesday, February 2, to commemorate the day that began the process of dismantling the apartheid system....
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If it makes you feel all warm and fuzzy to believe that boycotts bring down governments, fine. But what really brought S.A. down was pressure from the Anglo-American empire and their denial of funds from CIA-connected University Endowments (Harvard, for example). Little acts work on little actors.