The Reserve Army was a field army of the British Army and part of the British Expeditionary Force during the First World War. On 1 April 1916, Lieutenant-General Sir Hubert Gough was moved from the command of I Corps and took over the Reserve Corps, which in June before the Battle of the Somme, was expanded and renamed Reserve Army. The army fought on the northern flank of the Fourth Army during the battle and became the Fifth Army on 30 October. History Haig developed a concept of all-arms units of "cavalry and mobile troops" to capture a portion of the German defences and enlarge the foothold for later exploitation. Haig wrote training instructions for the cavalry in March 1916, in which he described a breach being made in the German lines and the cavalry and mobile troops rushing forward to create a bridgehead, obstructing German reinforcements. Infantry would have time to move up to relieve the cavalry in the bridgehead, which would then operate behind parts of the front where German infantry were still fighting and protect the main force by extending the flank. Haig disbanded the two cavalry corps on 3 March 1916 and distributed the divisions to the armies and the new Reserve Corps. Gough was appointed to command the Reserve Corps in April, which was renamed Reserve Army in June. There was some uncertainty over the role of the army, Kiggell, the BEF Chief of Staff writing on 4 June, "The area in which the Reserve Corps (sic) may be employed must be dependent on events and cannot be foreseen". Gough was told to train the cavalry in the all-arms concept and to convince cavalry officers of the effectiveness of cavalry, when co-operating with artillery and infantry. In 1996, Stephen Badsey wrote that the Reserve Army was organised as a conveyor belt, to exploit the success of the Fourth Army, with the 25th Division first, followed by two cavalry divisions and then the II Corps infantry divisions. Sheffield wrote that the all-arms concept was an imaginative response to the tactical problems of 1915 and anticipated post-war moves towards mobile warfare, but was ambitious, and the staff work and traffic control required to make it work, would have been a great burden on the inexperienced armies and staffs of 1916. "Gough's Mobile Army" was then made ineffective by a disagreement between Haig and Rawlinson about the plan