 As a brigadier-general, Capper was first made deputy inspector of the lines of communication before being given the post of Chief Engineer to the Third Corps. In July 1915 he was promoted to major-general and made chief engineer of the British Third Army. In October, following the deaths of several senior officers at the Battle of Loos, including Capper's younger brother Major-General Sir Thompson Capper, he was promoted to overall command of the 24th Division. Capper remained in command of the division for the next 18 months, including periods of heavy fighting at the Battle of the Somme, in which his son John was killed in action serving with the Royal Artillery. The division also spent extensive periods of time in other sections of the line and gained extensive battle experience at the cost of high casualties. As a reward for his service in command of the division, he was presented with the Commander's Cross of the Légion d 'honneur by the French government.
