Now that more and more military records are coming online it is becoming increasingly easy to trace the likely movements of your ancestor during the First World War - particularly if there is a relevant published history.
With Anthony's help in finding some key records on Ancestry and FindMyPast it is possible to trace the movements of James Humphries. He was the "J" who sent a postcard to Miss M. Rowe in 1909 - which was the starting point of a mystery post card investigation. In one way his case is simplified because he joined the Watford Battery of the 4th Herts Brigade R.F.A. of the Territorial Force - and almost certainly stayed with it all through the war - although it had been renamed the 270th Brigade R.F.A. by the end of the war. It is also helped because the history of the Hertfordshire soldiers have been well documented by J.D. Sainsbury, in this case in The Hertfordshire Batteries Royal Field Artillery. See James Humphries' Career