Upon the outbreak of World War I in August 1914, Miles, with his battalion, then serving in Dublin as part of the 13th Brigade of the 5th Division, was sent overseas to France, landing at Le Havre on 15 August, as part of the British Expeditionary Force (BEF). Promoted to the temporary rank of lieutenant on 27 August 1914, Miles, his rank of lieutenant made permanent, participated in all the major battles in which the battalion was engaged that year, beginning with the Battle of Mons and the subsequent retreat, and culminating in the First Battle of Ypres, where Miles's battalion suffered very heavy losses, although Miles himself survived unscathed and, from 18 November 1914, was seconded to the Army Signal Service. During the Battle of Hill 60 in April−May 1915, Miles, while attached to the 5th Signal Company, Royal Engineers, was awarded the Military Cross (MC) on 23 July 1915. The citation read: