The French were the first Europeans to enter the region when they began exploring the Congo basin in the mid-19th century. It was the Germans, however, who first gained formal control over the area, establishing the eastern border through negotiations with France between 1885 and 1908. Ironically, the earliest German colonists to the eastern forests were largely entering unknown territory. The Germans set to work building roads and establishing plantations, both endeavors requiring forced labor by the natives. This often led to violence, such as when the Gbaya under Mbartoua led a rebellion in the Bertoua region in 1903. Another revolt occurred when the Kaka protested their loss of lucrative trade opportunities that they had enjoyed in pre-colonial days.