I'll give an example. Can't really quote a book on this, but I'm Belgian so I'm at least somewhat familiar with Congolese history and I'll use that. See, you probably think of the Democratic Republic of the Congo as one nation with a state. I don't know how it was for other colonies (I expect much the same), but when the colonizers arrived there, there was nothing of the sort. What they found were hundreds of different tribes, with almost as many languages, living across vast distances. Many of these tribes had never encountered each other, a lot of them probably weren't aware of each other's existence. So while the Western colonizers saw a huge homogeneous mass of "uncultured natives", that would most certainly not have been the local experience.
This lack of unity was obviously important; later on a relatively small contingent of white people came to colonise and brutally oppress the local population. I believe estimates of the death toll in the Belgian Congo go from 4-20 million. Had the natives had a sense of nationality or membership of the same group they might have tried a Vercingetorix-style uprising, but a lack of shared identity made that difficult (coupled with other factors like the technological superiority of Europeans at the time).
Another, possibly better known example is of course the Rwandan Genocide, where racial tensions between the Hutus and Tutsis were stoked up, again by Europeans. If I recall correctly, the Hutus were farmers while the Tutsis were shepherds. The Europeans said the Tutsis were superior and so created a sort of racial hierarchy which tragically resulted in the genocide. I suppose you could sum this up under the old "Divide and conquer" maxim.