The British Reserve Army was formed in May 1916 and renamed as the Fifth Army in October 1916. It formed part of the British Expeditionary Force which fought in France and Flanders during WW1. Commanded by Hubert Gough in 1918 it failed to withstand a German offensive and withdrew giving up hard-won territory. The fifth army was ruined and was disbanded March 1918. Reformed under the command of William Birdwood on 31 May 1918 it was disbanded on 30 November 1918.
Alternatively: The Children's Newspaper of April 16, 1938, carries a report of a "fighting retreat of our Fifth Army" in March 1918 when "the war was lost and won", and an appeal for funds to endow two wards in St Mary's Hospital as a memorial to "the bravest men who ever saved the flag".
We can't find a picture of the Fifth Army. Our picture shows '4th Battalion, The Worcestershire Regiment, holding the 'La Creche' position on the Lys during a German attack, April 1918' - close enough?