Following the outbreak of World War I and the German invasion of Belgium, Tombeur became the commander of the Belgian colonial forces on the border with German East Africa. He presided over the re-organisation of the "Force Publique" and, in April 1916, led a Belgian force into Rwanda as part of the British-led East African Campaign. By June 1916, the whole of Rwanda had fallen and Burundi was occupied soon afterwards. Tombeur was made military governor of the Belgian Occupied East African Territories consisting of the future Belgian mandate of Ruanda-Urundi. The expeditionary force soon headed for the German military stronghold at Tabora in modern-day Tanzania. After a lengthy battle, Tombeur entered the town on 20 September 1916. Tombeur was recalled to Europe in late 1916 and was replaced as commander in East Africa by Armand Huyghé. In early 1917, he was promoted to the rank of Vice-Governor-General, the second highest administrative position in the Congo. He served as governor of Katanga from 1918 to 1920.