If one looks back over the decades since the end of WWII there have been many mass murders around the world. I believe part of Clinton's hesitation to commit troops to Rwanda had to do with Somalia. As American actions in Somalia made little difference, it was assumed that much the same would occur in Rwanda.
As for South Africa, apartheid isn't just politically incorrect, it's a modern caste system that discriminates against people because of their appearance (obviously). There are instances where such things occur around the world, but not to the extent that they did in South Africa. It was archaic and as the people in South Africa that were behind apartheid were of European descent and involved in various organizations and politics such as reputed ties to Israel, Brazil, and as a bulwark against communism in sub-Saharan Africa. During the Cold War allies were made on the basis of how useful they were and not how they ran their countries. Since South Africa wasn't a communist state it was given some leeway.
Again, the question can be asked: what could the US or the west have done that wouldn't involve sacrifice with no guarantees of success? Nothing.
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