According to Wikipedia: "

In the period from 1885 to 1908, many well-documented atrocities were perpetrated in the Congo Free State (today the Democratic Republic of the Congo) which, at the time, was a colony under the personal rule of King Leopold II of the Belgians. These atrocities were particularly associated with the labour policies used to collect natural rubber for export. Together with epidemic disease, famine, and a falling birth rate caused by these disruptions, the atrocities contributed to a sharp decline in the Congolese population. The magnitude of the population fall over the period is disputed, but it is thought to be between one and fifteen million.

At the Berlin Conference of 1884–85, the European powers allocated the Congo Basin region to a private charitable organisation run by Leopold II, who had long held ambitions for colonial expansion. The territory under Leopold's control exceeded 2,600,000 km2 (1,000,000 sq mi) and, amid financial problems, was ruled by a tiny cadre of administrators drawn from across Europe. Initially, the colony proved unprofitable and insufficient, with the state always close to bankruptcy. The boom in demand for natural rubber, which was abundant in the territory, created a radical shift in the 1890s—to facilitate the extraction and export of rubber, all "uninhabited" land in the Congo was nationalised, with the majority distributed to private companies as concessions. Some was kept by the state. Between 1891 and 1906, the companies were allowed to do whatever they wished with almost no judicial interference, the result being that forced labour and violent coercion were used to collect the rubber cheaply and maximise profit. A native paramilitary army, the Force Publique, was also created to enforce the labour policies. Individual workers who refused to participate in rubber collection could be killed and entire villages razed.

Despite these atrocities, the main cause of the population decline was disease. A number of pandemics, notably African sleeping sickness, smallpox, swine influenza, and amoebic dysentery, ravaged indigenous populations. In 1901 alone it was estimated that 500,000 Congolese had died from sleeping sickness. Disease, famine and violence combined to reduce the birth-rate while excess deaths rose.

The severing of workers' hands achieved particular international notoriety. These were sometimes cut off by Force Publique soldiers who were made to account for every shot they fired by bringing back the hands of their victims. These details were recorded by Christian missionaries working in the Congo and caused public outrage when they were made known in the United Kingdom, Belgium, the United States and elsewhere. An international campaign against the Congo Free State began in 1890 and reached its apogee after 1900 under the leadership of the British activist E. D. Morel. In 1908, as a result of international pressure, the Belgian government annexed the Congo Free State to form the Belgian Congo, and ended many of the systems responsible for the abuses. The size of the population decline during the period is the subject of extensive historiographical debate, and there is an open debate as to whether the atrocities constitute genocide. Neither the Belgian monarchy nor the Belgian state has ever apologised for the atrocities