 The army fought on the northern flank of the Fourth Army during the battle and became the Fifth Army on 30 October.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.