I think you're approaching this from a slight misconception. I can only answer from the perspective of the British Army, but I think your implication that important commands were given to prestigious aristocrats because of their social status is wrong. This type of question on the British Army isn't really my area of expertise, and I don't have access to the primary documents I'd like to use to answer it fully, but I have a couple of good secondary sources to hand and can give an imperfect but nonetheless well-sourced answer on that basis. I'll be answering your question from the British perspective with an assessment of the pre-1914 Regular Army and comparing that to the way the British Army developed during the war.

In Tommy, Richard Holmes makes the point that Wellington would have been struck by how similar the officer corps of 1914 was to that of the army he commanded at Waterloo. He quotes Edward Spiers by saying that the British Army's officers came from the "traditional sources of supply". By this, he means that most officers came from family backgrounds in the peerage, gentry, long-standing military families, the clergy and the traditional professions (ie: medicine, the law, engineering, architecture etc.). But, he also points out that social divisions were not concrete, and for lots of officers this background did not equate to great personal or familial wealth. Many officers struggled to support themselves and the lifestyles their regimental duties required, and in many cases a military career was a stepping-stone in a family's attempt to improve its position on the social ladder; a family which had made money in business, commerce or industry would buy land, marry its daughters into the aristocracy and send one or more of its sons into the Army to set the seal on its gentility. So, even an apparently wealthy young officer with a good education and a commission in a smart regiment might only be a generation or two away from a very average lifestyle.

The British Army had abolished the purchase of commissions as part of the Cardwell Reforms in 1871, so from then on it was no longer possible to pay a certain amount of money and receive a commission as an officer. There are lots of misconceptions about this practice which are beyond the scope of this response, but suffice it to say by 1914 wealth was far from a determining factor in who could serve as an officer in the British Army.

The usual route to a commission in the Edwardian Army (ie: 1901-1914) was via entry to one of two officer training institutions operated by the British Army. Officers for the Royal Artillery and Royal Engineers were trained at the Royal Military Academy, Woolwich. Entry was by competitive examination and the entrance exam included compulsory papers in English, French or German, and mathematics, and a choice of any two further papers from further mathematics, history, German, Latin, French and science. So, you can see any aspiring gunner or sapper officer would need to pass an exam in at least one foreign language as well as supplementary language or technical exams. The most successful graduates from Woolwich tended to join the Royal Engineers, and the remainder the Royal Artillery. The December 1909 cohort had 36 members, and numbers 1-11 joined the sappers and the remainder the gunners. Holmes cites the example of Alan Brooke, the future Field Marshal 1st Viscount Alanbrooke, Chief of the Imperial General Staff (CIGS) between 1941 and 1946. He "sweated blood" with an Army crammer and only barely qualified to attend Woolwich, coming 65th out of 72 in the entrance exam. He later passed out 17th of his cohort and was commissioned into the Royal Artillery.

Officers for the cavalry, infantry, Indian Army and Army Service Corps were trained at the Royal Military Academy, Sandhurst. Again, entrance was by competitive examination and your place in the final order of merit determined where you would serve. Due to the lower cost of living in India, officers in the Indian Army were able to live on their pay - more on the expense of being an officer in the pre-war Regular Army later - and anyone hoping to secure one of these desirable commissions needed to pass out towards the top of his class.

Sandhurst only trained approximately 67% of cavalry or infantry officers (compared to Woolwhich training 99% of gunners and sappers). Around half of the remainder were commissioned from the universities thanks to the training they had received in the Senior Division of the Officers' Training Corps.

It was also possible to enter the Regular Army via service in the militia or Yeomanry (or, from 1908, the Special Reserve or the Yeomanry). Without going into too much detail, the Militia was the volunteer reserve of the Regular Army in which recruits signed on for a set period of service. It was loosely regulated until 1908 when the Special Reserve replaced the Militia. At the same time, the Territorial Force (TF, later the Territorial Army (TA)) was set up. The Special Reserve was separate from the TF. It was composed of volunteers who signed up for six years of full-time service. They undertook six months Regular training and a further four weeks' training annually. The creation of the Special Reserve led to the creation of new Special Reserve infantry battalions which were integrated into the Regular Army's regimental system; regiments with two battalions (the majority of the county regiments like the Royal East Kent Regiment) received a 3rd (Special Reserve) Battalion and four battalion regiments (such as the King's Royal Rifle Corps) received 5th and 6th (Special Reserve) Battalions. They were intended to form an immediate pool of manpower for the Regular Army, and were to be brought up to War Establishment by members of the Regular Reserve, former Regular soldiers who remained liable to be recalled to the colours for a certain period after their Regular service ended. The TF were part-time volunteers who joined cavalry, infantry, artillery and engineer units intended for home defence and with a nebulous pre-war liability for overseas service. The Yeomanry were part-time volunteer cavalry regiments which were part of the TF, not the Special Reserve, but because they were the only reserve cavalry units in the British Army it was possible for Yeomanry officers to join the Regular Army in the same way as infantry officers of the Special Reserve. Officers of the Militia/Special Reserve and the Yeomanry were able to transfer to the Regular Army without entering Sandhurst after passing a competitive examination for a direct commission. Field Marshal Sir John French (who had begun his career in the Royal Navy and would command the British Expeditionary Force in 1914-1915) and Field Marshal Sir Henry Wilson (French's Deputy Chief of Staff in the BEF in 1914 and CIGS in 1918) both secured their commissions in this way.

Around 2% of officers in the pre-war Regular Army were commissioned from the ranks to serve alongside officers who had come through Sandhurst/Woolwich, the OTC or militia/Special Reserve/Yeomanry. These were Regular commissions granted for service on the battlefield and they would be granted on the same terms (and with the same seniority) as any other Regular commission. They were separate from the commissions granted to Quartermasters or Riding Masters, who were commissioned from the ranks but given honorary commissions to undertake specific jobs due to their technical expertise. At the outbreak of the First World War, Field Marshal Sir William Robertson was serving as CIGS. He had joined the Army as a private in the 16th Lancers in 1877, and after being promoted to sergeant major in 1885 he passed the examination for a direct commission into his regiment.

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