How were slaves essential to all 3 sides of the trade?

Mercantilism led to the emergence of what’s been called the “triangular trade”: a system of exchange in which Europe supplied Africa and the Americas with finished goods, the Americas supplied Europe and Africa with raw materials, and Africa supplied the Americas with enslaved laborers.

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if you're talking about (ww2) they went bankrupt and their currency was worth nothing

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this question shoud be under spanish category, not