A watershed for the British military was World War One where casualty rates outstripped the supply of upper and upper-middle class volunteers. The English private school system prided itself on supplying the Army with officers and the (relatively) small pre-war army had an officer corps drawn almost exclusively from private schools and a handful of respectable grammar schools with many having a degree form a decent university^*. After the outbreak of war current and former private school boys joined up in their droves, inspired by the highly patriotic sense of duty which was a feature of the curricula at these schools.

During 1915 the supply of volunteers started to wane, with conscription being introduced in March 1916. The small number of “proper” schools, high numbers of early volunteers and high casualty rate amongst junior officers meant that the traditional schools alone could not meet the demand and so the Army had to reconsider its approach. This process was aided by a reduction in snobbery amongst the officer corps as junior officers from privileged backgrounds lived and died alongside working class men they would otherwise not have met. Those junior officers who survived to be promoted took this slightly more relaxed attitude with them and the Army became gradually more meritocratic. Young soldiers with leadership potential were more likely to be spotted and many experienced sergeants major made excellent candidates for commissions in support roles (e.g. quartermaster) freeing up younger officers for more demanding work closer to the front. The process also varied between regiments. Cavalry and guards officers remained almost exclusively upper class as commissions in these regiments were in very high demand, whereas county and territorial regiments had to diversify.

It also became apparent that recruits with relevant skills made good officers in technical branches. There was little to be gained from putting an Old Harrovian in charge of a company of miners when an ex-colliery foreman would do a much better job. The arrival of the Royal Flying Corps and ultimately the Royal Air Force also put a middle class cat amongst the posh pigeons as it transpired that flying ability made no allowances for class. Equally the more mathematically gifted could find themselves commissioned in the artillery or a construction manager into the Engineers. The great benefit of a citizen army is that recruits bring with them all of the skills they had in civilian life and the Army learned to take advantage of this. The BEF became the largest population of Britons outside of metropolitan London and so there was plenty of less glamorous leadership required from those of less glamorous descent!

The British Expeditionary Force also included large numbers of dominion and commonwealth soldiers who brought their own cultures with them. Australian and New Zealand troops were notably egalitarian, demanded to be led by officers with talent rather than pedigree and were generally dismissive of British class-based traditions. For this reason William Birdwood was chosen to form and lead the Australia and New Zealand Army Corps (ANZAC) as he deliberately avoided the heirs and graces of a typical British officer. He was not afraid to talk to soldiers in their own language, cared little about dress standards or enforcing customs such as saluting and routinely exposed himself to danger e.g. bathing in the sea at Gallipoli whilst under Turkish shell fire. The ANZACs loved him for it.

The transition was not total. The reduction of the Army after the war brought it back within the influence of the private schools, but society had become less stratified by the war and it became increasingly difficult for any institution to survive on class-based distinctions alone.

^* Commissioning from the ranks was possible for those who distinguished themselves. Field Marshal Robertson was The Chief of the Imperial General Staff from 1916 to 1918 and is the only person in British history to have held every rank from private to field marshal.