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They didn't really take it, they were kind of forced to buy it. At a high price no less, even as the "profitability" was already in steep decline. Congo had lots of wild rubber trees, which the colonies of the competition (the other Western countries) didn't have. Rubber tree plantations took a few decades (iirc) to become operable. Leopold II took full advantage of the window of opportunity to extract as much wealth and resources as possible, using ruthless overseers that commited incredible atrocities. The situation got so extreme that it caused criticism and condemnation from all over the world, which was unique at the time. Leopold II was able to flatter, bribe, lie and blackmail his way to his own colony and the means to keep it for years. He was loaned lots of money by the Belgian government during the time he had the colony. He even managed to convince the international community that he was a philanthropist caring deeply about the people he was in reality abusing in every way imaginable. By the time the Belgian government bought the colony, the first plantations were already in operation and more importantly, the condemnation of the scandals and the violence made Leopold holding onto the Congo untenable. This is not to say the Belgian government was somehow innocent, the atrocities did not immediately stop after the transfer of ownership, and even before a lot of Belgians had benefited from the colony in some way. But by far the lion's share of the riches stolen from the Congo went straight to Leopold as the sole owner and ruler of the Congo. He used this incredible wealth to fund megalomanic extensions to the royal palace and other buildings in his possession, such as giant mansions in France. Iirc he didn't even pay off his debts to the Belgian government.

I could go on but this comment already got way too long. For anyone interested, King Leopold's Ghost by Adam Hochschild is a fantastic read. As a Belgian, I think this should be mandatory reading, it's crazy how we hardly learn any of this stuff.