The former Belgian Congo was once the mighty Kongo empire. Unfortunately, Portugal and other European countries saw the empire as a bountiful source for slaves. So many were taken that the Kongo was severely weakened and eventually collapsed by the late sixteenth century.
King Leopold II of Belgium made the area into a privately owned kingdom in the late nineteenth century. Leo's reign was a basic terror state, with an estimated 3-21 million people being killed.
International outrage over Leo's atrocious reign lead to the area being annexed by Belgium in 1908 as the "Belgian Congo." Unfortunately, things didn't improve much for the Congolese. Belgium showed no respect for native culture, instead imposing Christian and Western values. The Belgian Congo came to an end in 1960.
Belgian Congo on Wikipedia