Brooke was commissioned into the Royal Regiment of Artillery as a second lieutenant on 24 December 1902. During the First World War, he served with the Royal Artillery in France where he gained a reputation as an outstanding planner of operations. At the Battle of the Somme in 1916, he introduced the French "creeping barrage" system, thereby helping the protection of the advancing infantry from enemy machine gun fire. Brooke was with the Canadian Corps from early 1917 and planned the barrages for the Battle of Vimy Ridge. In 1918 he was appointed GSO1 as the senior artillery staff officer in the First Army. Brooke ended the conflict as a lieutenant-colonel with the Distinguished Service Order and Bar.