As he was too young, Ritchie was not immediately sent overseas until after his 18th birthday in July 1915. He was promoted to lieutenant on 2 October, and served initially with the 1st Battalion, Black Watch, then part of the 1st Brigade of the 1st Division and one of the two Regular Army battalions in the regiment, in the trenches of the Western Front, where he was wounded during the Battle of Loos. After recovering from his injuries he was promoted to the temporary rank of captain on 2 March 1916, and made an acting captain on 22 April. He later served with his regiment's 2nd Battalion, part of the 21st (Bareilly) Brigade in the 7th (Meerut) Division and the second of his regiment's two Regular Army battalions, in the Sinai and Palestine campaign and the Mesopotamian campaign, including in the fall of Baghdad in March 1917. On 5 January 1917 he became his battalion's adjutant. He won the Distinguished Service Order (DSO) on 25 August 1917 and, having been confirmed in his rank of captain on 19 November 1917, was awarded the Military Cross (MC) on 15 February 1919 for his actions during the Battle of Megiddo in September 1918. The citation for his MC reads: