After WWI, priority was given to mining (copper and cobalt in Katanga, diamond in Kasai, gold in Ituri) as well as to the transport infrastructure (rail lines Matadi-Léopoldville and Elisabethville-Port Francqui). To obtain the necessary capital, the colonial state gave the private companies, to a large extent, a free hand. This allowed, in particular, the Belgian Société Générale to build up an economic empire in the colony. Huge profits were generated and for a large part siphoned off to Europe in the form of dividends.[19] The necessary work force was recruited in the interior of the vast colony with the active support of the territorial administration. In many cases, this amounted to forced labour, as in many villages minimum quotas of “able-bodied workers” to be recruited were enforced. In this way, tens of thousands of workers were transferred from the interior to the sparsely populated copper belt in the south (Katanga) to work in the mines. In agriculture, too, the colonial state forced a drastic rationalisation of production. The so-called “vacant lands”—i.e., the land that was not directly used by the local tribes—fell to the state, which redistributed it to European companies, individual white landowners (colons) or the missions. This way an extensive plantation economy developed. Palm oil production in the Congo increased from 2,500 tons in 1914 to 9,000 tons in 1921 and 230,000 tons in 1957. Cotton production increased from 23,000 tons in 1932 to 127,000 in 1939.[20] After WWI the system of mandatory cultivation was introduced: Congolese peasants were forced to grow certain cash crops (cotton, coffee, groundnuts) destined for the European market.
The economic boom of the 1920s turned the Belgian Congo into one of the leading copper ore producers worldwide. In 1926 alone, the Union Miniére du Haut Katanga exported more than 80,000 tons of copper ore, a large part of which was processed in Hoboken in Belgium.
During WWII, industrial production increased drastically. After Malaysia fell to the Japanese, the Belgian Congo became a strategic supplier of rubber to the Allies. The Belgian Congo was one of the major exporters of uranium to the US during WWII (and the Cold War), particularly from the Shinkolobwe mine.