From "Sanctions and South Africa" (January 1988), a publication of The Economist Intelligence Unit, by Merle Lipton:

The intense media coverage from late 1984 contributed to the bankers' flight and imposition of formal sanctions, thereby intensifying verkrampte demands for the clampdown on the press and on political activity which has since taken place. This has destroyed the limited and precarious, but valuable, political space that was emerging for the opposition and which has now practically disappeared, reducing the possibilities for black political organization and for public debate across racial lines.

The role of external pressures in this should not be overstated -- it was the internal unrest that was decisive. But, in a fluid situation, what often appear to be marginal influences can be important in pushing developments in one direction rather than another. There seems to be a threshold beyond which the initially often helpful effects of external pressures become counter productive.

. . . Increased sanctions are thus among the factors reducing the "political space" available for all South Africans, white as well as black. This could impede the interracial political mobilization and debate that was under way and reduce the chances of an evolutionary route to a post-apartheid South Africa. Continued incremental sanctions seem unlikely to unseat the government, and more likely to impede than to accelerate reform (i.e., deracialization). They are also likely to ensure that this takes place in an even more authoritarian context. The impact of a drastic program of comprehensive sanctions must be adjudged very uncertain.