British Army First World War reserve brigades The British Army First World War reserve brigades . At the start of the First World War British Army volunteers in the vast majority of cases joined their local infantry regiment s reserve battalion . In 1916 when conscription was first introduced for the British Army , the existing regimental system could not cope with the large influx of recruits . By January 1916 , when conscription was introduced , 2 . 6 million men had already volunteered for service , a further 2 . 3 million were conscripted before the end of the war ; by the end of 1918 , the army had reached its peak strength of four million men . To train the recruits from 1916 onwards twenty - six reserve brigades were raised , with an official complement of over 208 , 500 soldiers . Background . In August 1914 , 300 , 000 men had signed up to fight , and another 450 , 000 had joined - up by the end of September . Recruitment remained fairly steady through 1914 and early 1915 , but it fell dramatically during the later years , especially after the Somme campaign , which resulted in 360 , 000 casualties . A prominent feature of the early months of volunteering was the formation of Pals battalion s . Many of these pals who had lived and worked together , joined up and trained together and were allocated to the same units . The policy of drawing recruits from amongst the local population ensured that , when the Pals battalions suffered casualties , whole towns , villages , neighbourhoods and communities back in Britain were to suffer disproportionate losses . With the introduction of conscription in January 1916 , no further Pals battalions were raised . Conscription for single men was introduced in January 1916 . Four months later , in May 1916 , it was extended to all men aged 18 to 41 . The " Military Service Act March 1916 " specified that men from the ages of 18 to 41 were liable to be called up for service in the army , unless they were married ( or widowed with children ) , or served in one of a number of reserved occupation s , which were usually industrial but which also included clergymen and teachers . This legislation did not apply to Ireland , despite its then status as part of the United Kingdom ( but see Conscription Crisis of 1918 ) . By January 1916 , when conscription was introduced , 2 . 6 million men had volunteered for service , a further 2 . 3 million were conscripted before the end of the war ; by the end of 1918 , the army had reached its peak strength of four million men . Reserve Brigades . At the start of the First World War , most British Army infantry regiments had their own reserve battalions . After conscription was introduced from the 1 September 1916 onwards , the reserve infantry battalion were reorganised . The previous regimental system simply could not cope with the influx of recruits and a centralised training and reserve system was introduced . The reserve battalions still kept their regimental affiliations , cap badge s and accoutrements , until 1 September 1916 , when the regimental distinctions disappeared and the battalions were re - designated as the 1st to 116th reserve battalion in one of twenty - six reserve brigades . The Training Reserve had an official complement of 208 , 500 soldiers . Another consequence of the centralised training system , was when recruits were posted to an battalion on active service , they would be sent to where there was a vacancy and not his local regiment . This system lasted until May 1917 , when the reserve battalions were once again affiliated with a particular regiment and became known as graduated or young soldier battalions of their regiment .