1914–1915
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 HMS Bacchante.
When the Gallipoli campaign started, General Alexander Godley, formerly of the Irish Guards and second in command to General Birdwood of the Australian and New Zealand Army Corps (ANZAC), now commanding the New Zealanders, offered him an appointment as liaison officer and interpreter on the general's staff.
His pre-war contacts (including Rıza Tevfik Bölükbaşı) and ability to speak Turkish were to prove useful. He became famous for arranging a truce of eight hours, on Whit Monday, 24 May, with the Turkish commander Mustafa Kemal, for the purpose of burying the dead. This episode appears, with him as "the Honourable Herbert", in Louis de Bernières's novel. In Eastern Mediterranean Intelligence, he worked with Compton Mackenzie.
In October 1915, on sick leave in England, Herbert carried with him a memorandum, stating in part: "They [Foreign Office] trust Egypt with the running of the Arabian Question...", from the Cairo Intelligence Department from Colonel Clayton to the Foreign Office explaining the situation in the Middle East. In November, the memorandum, at first favourably received, became obsolete after the visit of François Georges-Picot and his subsequent negotiations with Mark Sykes. It would appear that the Arab Bureau continued working along the lines of the memorandum, which led to contrary promises ensuing accusations of bad faith.
In November 1915, Herbert was in Paris and Rome on a secret mission related to Albania. Following the plan to evacuate Anzac Cove beginning the following month, he volunteered to return to ANZAC to stay with the rear guard, convinced that his knowledge of language and his network of acquaintances would greatly benefit that body if captured. The 20 December successful evacuation of ANZAC and Suvla Bay and the good prospects for Cape Helles countered his proposal.