It's not clear that this was initially true with all or most of Africa. I recall reading in, I think Basil Davidson that the Portugese were astounded by the wealth and power of the African kingdoms they first visited in the early 16th century in -- if I recall accurately -- Mali. America is a different story, where the Europeans faced technologically relatively unsophisticated if (in South and Central America) socially complex societies.
However, the question of how the Europeans were able to defeat and colonize others is not the same as whether the reproduction or prosperity of the nascent capitalist systems that developed in Europe in the last 500 years dependind on the exploitation of the colonies. By analogy, the barbarian Huns, etc. generated the military tactics and technolofy that enabledthem to defeat the socially far more sophisticated Romans, but the survival and prosperity of their social system did not depend on the loot they took from their victims. It was icing.
I should also note that it's a mistake to say of someone like Brenner, who doubts that either the development or the present success of capitalism depended essentially on slavery and imperialism and colonialism, thereby thinks that these things :don't matter," or to suggest that he offersa pologetics or justifications of slavery and imperialism, or even of capitalism. The causal relations are what they are. No doubt capitalism swept up human beings in its early drive to commodify everying, although slavery is a much older institution than capitalism. No doubt the early capitalist societies were colonialist and imperialist. No dount, European societies profited from slavery and from coloniala nd imperialist theft and rapine. That does not mean that early capitalsim or present capitalism depends on slavery, imperialism, and colonialism in that they would collapse or be unable to reproduce the system without it.
jks
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