Despite very poor eyesight, Herbert was able, at the outbreak of World War I in 1914, to join the Irish Guards, in which he served in a supernumerary position. He did this by purchasing a uniform and boarding a troopship bound for France. During the Battle of Mons, he was wounded and briefly taken prisoner. After a convalescence in England, and unable to rejoin due to his ocular disability, Aubrey was proposed for service in military intelligence in Egypt by Kitchener's military secretary, Oswald FitzGerald, via Mark Sykes (see Baghdad Railway). In January 1915, Herbert was attached to the Intelligence Department in Cairo under Colonel Gilbert Clayton. In mid-February, he was sent on an intelligence mission into the eastern Mediterranean aboard the cruiser .