With the outbreak of the First World War, he returned to active duty, receiving a promotion to substantive lieutenant-colonel in November 1914. He first served as assistant military secretary at the War Office, and from December 1915 as military secretary to the commander-in-chief of the British Expeditionary Forces (BEF) in France, Sir Douglas Haig, with the temporary rank of brigadier general. He received from allied nations the Belgian Order of Leopold (Grand Cordon) and Croix de guerre, as well as the French Legion of Honour (Grand Officer).