Sykes was made Assistant Adjutant and Quartermaster-General of the 4th Mounted Division in March 1916 and, having been awarded the Russian Order of St Vladimir, 4th Class on 12 April 1916, he was appointed Assistant Adjutant-General at the War Office with responsibility for organising the Machine Gun Corps and manpower planning on 9 June 1916. He was made Deputy Director of Organisation at the War Office and granted the temporary rank of brigadier general on 8 February 1917. On 27 November 1917 he became Deputy Adjutant and Quartermaster-General at the War Office in which role he served on the British section of the Allied War Council in the Palace of Versailles under General Sir Henry Wilson.