A well-organized minority with superior firepower can dominate an unorganized minority for a long time.
Also, there is the old tactic of dividing the populace over some issue, and setting them against each other, so they won’t unite and turn on you (see Barack Obama setting blacks against whites here in America for an example of that). I imagine that tactic was probably very effective in SA, where there were already pre-existing tribal divisions.
Whites had the guns and the British Empire supported the Boers after the Boer war until the 70’s. Easy enough
It is a fallacy that a minority can’t dominate a majority for long. Simply untrue.
What is impossible is for a minority infected with modern Christian and secular ideas of equality to do so. For a people with no ethical inhibitions, no problem.
Perhaps the most dystopian SF series I’ve ever read addressed the South African situation, assuming that white South Africans decided to go the non-ethical route, based on a somewhat different history. Wind up conquering and enslaving the entire world, not just Africans.
Very nasty. Recommended if you have a somewhat strong stomach.
https://en.wikipedia.org/wiki/The_Domination