Let me start off by saying that it’s important to note that the timeframe from the Crimean War to World War I was a time of rapid military advancement. In that time, militaries went from Napoleonic weapons and tactics to dispersed formations and and automatic weapons, so pretty much any conflict saw a new technology or idea being used. As such, anything and everything was being reported.
The ACW for example saw a fair share of interest from the British. Officers often time took leave to observe the conflicts and what they wrote about influencers various fields of the British military. An example of this is that Canada saw two reviews of its military defense during the duration of the war.[1] The British observers also wrote about the changes in tactics. The old dense formations were rendered obsolete by the increase in firepower and infantry now had to disperse or take cover behind increasing amounts of fortifications.[2]
To see the level some of these reports could go into, one has but to look at the various sections in Reports on Military Operations in South Africa and China written by US military personnel. In the China portion, there are reports for every nation that participated in the Boxer Rebellion. To use the report on the Japanese as an example, it looks at how many and what type of entrenching tools were carried( 2/3 of the infantry carried a shovel with a 7 in. by 7 in. blade and 1 foot handle), rations used (36 ounces of rice, 4 ounces of meat, and 4 ounces of veggies), and even pay (corporals got paid 1 yen 80 sen in war time).[3] such reports helped to serve as a gage for where one’s own military was in relations to others and if there was anything that could be adopted.
Moving on to the Russo-Japanese War, this got a lot of attention from both observers and non-observers. Probably one of the more famous non-observers was Sir Julian Corbett who wrote the British War Offices official report on the naval action of the conflict. Six years later, Corbett would publish Some Principles of Maritime Strategy, one of the great treatises on naval warfare, alongside A.T. Mahan’s The Influence of Sea Power upon History: 1660–1783. Other works by non-observers included “Lessons to be Learnt from the Siege of Port Arthur as Regards R.E. Work” in the Journal of the United Service Institution of India and “A New Tactical System applied to the Russo-Japanese War” published by Norwegian Admiral Jacob Børresen in a British journal. In regards to the actual observers, it should be noted that many went on the serve in key positions during WWI, most famous of which is undoubtedly John Pershing.
It should also be noted that observers were not just limited to wars. On the American side, one of the more famous post-ACW observers was Emory Upton. One of his earliest observations of foreign militaries was when he left his ailing wife during their honeymoon in France to see maneuvers of the French military.[4] However, his most famous observations started in 1872 when he and a group of other US Army officers left California to study the militaries of various nations. His trip took him to Japan, China, British India, Persia, Russia, Italy, Germany, Austria-Hungary, France, and England. He then published his findings in The Armies of Asia and Europe, 1878.
[1] Somerville, Michael Bull Eun to Boer War: How the American Civil War changed the British Army, pg 42.
[2] Somerville, Bull Run to Boer War, 210.
[3] Major Muir, Charles “Reports on Japanese Troops in North China” in Reports on Military Operations in South Africa and China, pg 359-360.
[4] Fitzpatrick, David, Emory Upton: Misunderstood Reformer, pg 100.