By November 1914 he was commander of 1st Wing RFC with the rank of lieutenant colonel. The following month he was appointed Officer Commanding Administration Wing RFC. Promoted to brigadier general in January 1916, he took command of the I Brigade RFC. The following April he took up command of the newly established IV Brigade. He remained attached to RFC Headquarters unit until August 1917 carrying various ranks including brevetcolonel, acting major general, colonel and major general. In August 1917 Ashmore was appointed Commander of the London Air Defence Area. When the Royal Flying Corps amalgamated with the Royal Naval Air Service to form the Royal Air Force in April 1918, Ashmore still held the rank of major general. On 19 April 1918 he was re-graded to air vice marshal until he resigned his RAF commission on 1 December 1919.
Air defence of London
In his later military career, Ashmore was the leading figure in the air defence of the United Kingdom, founding what would eventually become the Royal Observer Corps. He was appointed to devise improved systems of detection, communication and control. A system to be called the Metropolitan Observation Service was created which covered the London area and was known as the London Air Defence Area. This was soon extended towards the coasts of Kent and Essex. The system met with some success and although it was not fully working until late summer 1918. The lessons learnt were to provide valuable grounding for later developments.
Interwar
On 8 December 1920, Ashmore was appointed commander of the Regular Army's 1st Air Defence Brigade in Aldershot Command. In 1922, overall responsibility for air defence was transferred from the War Office, which was responsible for the Army, to the Air Ministry. Ashmore, who had been responsible for matters during the First World War, now reported to a new Air Raid Precautions committee set up in January 1924. On 1 March 1924 he became General Officer Commanding of the Territorial Army Air Defence Brigades and Inspector of Regular Anti-Aircraft Defences for Great Britain. Experiments were now carried out around Romney Marsh and the Weald. These were intended to optimise the arrangement of observation posts and control centres. In 1925 these experiments were extended to cover parts of Essex and Hampshire and by October a sound methodology had been worked out. On 29 October 1925 the Observer Corps came into official existence. Within a year four Groups existed in Southeast England, covering much of Kent, Sussex, Hampshire and Essex. The plan was that the country would be covered by 18 of these groups. The involvement and cooperation of the RAF, the Army, the British police forces and the General Post Office , was required. Ashmore was regarded as the first commander of the Observer Corps in all but name during 1925 and he is deemed to be the Corps' founder.
