Hi u/OpenWaterRescue, thanks for the great question.
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A note before we begin, in modern historical and political scholarship we tend to avoid using phrases like "tribes", especially in the African context, as they rarely provide an accurate descriptor of the ethnic group in question as the phrase tends to bring with it certain baggage and biases which we would rather avoid And, I think you'll agree, considering that the Xhosa and Zulu ethnic-language populations consist of 8 million people and 14 million people respectively, I think "tribe" hardly does them justice just in terms of sheer numbers and their geographical diaspora across southern Africa. You can read more about why we don't use the term here: https://www.learningforjustice.org/magazine/spring-2001/the-trouble-with-tribe
However, "tribe" is also a very important descriptor here as it is highly relevant to the ethno-politics of colonialism and Apartheid ideology and policy - we'll unpack it in this answer somewhat. But fully unpacking ethno-linguo-cultural terminology in South Africa is a a can of worms that we can open up another day, for now to avoid confusion I'll just refer to them as the Xhosa and Zulu where relevant and will discuss tribes and tribalism as well (just bear in mind we tend not to use it when referring to the modern population groupings).
("Native", too, is not an ideal word to use in such a discourse for similar reasons, but we'll leave that aside for now.)
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As with everything regarding Apartheid history, we have to take a look at its foundation: colonialism. Early colonists found that one of the best methods of ensuring compliance from the ethnic groups they were attempting to subdue was first to define them along distinct tribal lines which included cultural and geographical spheres (regardless of if their actual ethnicities, lineages, and diaspora were more complicated than that).
They did this through multiple methods, including trade, religion, education, conflict, and political society. For example, by ensuring one group received preferential trade over another, it could incite resentment which could be flared and directed against groups as needed by the colonists. Education and religion too, could be used to "teach" the "savage native" about their "place" in the world ensuring compliance, and encourage learned helplessness or, again, ferment tensions by either installing the "us vs them" mentality of tribalism, or fanning the flames of existing tensions. And in terms of political society, colonists found that setting up chiefs of particular tribes as gatekeepers through which their people to gain resources from the colonial state.
"Ethnicity was the lynchpin of colonial rule. Colonial bureaucracy promoted ethnic identification as a dominant medium of interaction between itself and the individual ‘natives’. Access to state resources hinged on membership in one ethnic group or another. This was premised on the definition of ‘natives’, not as citizens within a nation-state, but as subjects that belonged to a particular ethnic community under a traditional authority. Thus a native could not lay any claims or demands for resources, shelter or security, from a colonial state, but from a traditional authority. The latter, in turn, demanded identification with that ethnic community and allegiance to its authority figure – a chief (Mamdani 1996:62-108)."(1)
[I would just like to stress, though, that despite how I phrased it above that the people who were colonised in Africa were anything but passive receivers of colonialism. That is an oft repeated myth which should be easily dispelled if you look into the Anglo-Zulu war or the Xhosa Wars, for example. But there is only so much that can be done when overwhelming military might is brought to bear and your people and land are systematically dismantled.
Further, it would also be wrong to place all ethnic group tensions at the feet of colonialism and Apartheid - such tensions long predated colonialism and the history of conflicts between ethnic groups is well understood. The Bantu Expansion is a prime example.]
There's a lot more that can be said to explain the build of ethnic tensions, but we should now move to the primary focus of the question: the Xhosa, Zulu, ANC, and Apartheid.
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