Du Cane served in the Second Boer War, and was appointed a staff officer for lines of communication in South Africa in September 1900.[5] Following the end of hostilities in early June 1902, he left Cape Town on board the SS Assaye,[6] and arrived at Southampton the next month. He was mentioned in despatches and received a brevet promotion to lieutenant colonel in the South Africa honours list published on 26 June 1902.[7]
After serving as a brigade major, Royal Artillery, he then served at the Staff College, Camberley, as a deputy assistant adjutant general (DAAG) from April 1905−1907.[8][9] He was promoted again to major in January 1908,[10] and full colonel later that month.[11] He then became a general staff officer, grade 1 (GSO1) at army headquarters.[12]
Du Cane served in the First World War initially as a brigadier general on the general staff of III Corps of the British Expeditionary Force (BEF).[1] In 1915, after being promoted to major general in February,[15] as major general, Royal Artillery, he was an artillery advisor at General Headquarters; Lieutenant General Sir William Robertson, chief of staff to the BEF in 1915, later stated that he had laid the organisational groundwork for the massive expansion of BEF artillery during the war.[16][1] He was posted to the Ministry of Munitions in 1916 and then, after being promoted to temporary lieutenant general,[17] became general officer commanding (GOC) of XV Corps in September 1916, after Sir Henry Horne went to command the First Army.[1] In that capacity, he was closely involved in Operation Hush, a planned invasion on the Belgian coast.[18] On 12 April 1918, against the backdrop of the German "Georgette" Offensive and Field Marshal Sir Douglas Haig's demands for French reinforcements, he was appointed liaison officer between Haig and the Allied generalissimo, Ferdinand Foch.[19]