I know that the Russo-Japanese War of 1904/05 is widely (at least in Germany) regarded as some kind of prototype of the First World War (for example a mechanized war, use of machine-guns, minimal use of cavalry if not the complete dismissal of it, heavy use of artillery) and that the major powers in Europe could have learnt quite a bit from it. What I'd like to ask you, AskHistorians, is:
Thank you in advance!
A great deal was learned. Unfortunately, the wrong lessons were learned. Elan and courage by massed infantrymen had prevailed (albeit) at massive cost during the Russo-Japanese War, so European commanders thought the same principles would prevail on a European battlefield.
http://www.historynet.com/russo-japanese-war-japans-first-big-surprise.htm
Decisive Naval encounters
Im not too sure on this, I dont think i've read books that have explicitly stated either side used chemical weapons
Yes. After the humiliation of the Russo-Japanese War, Russia tried to reform their military. This is one of the reasons why the Russian military mobilized incredibly quickly, throwing the German calculations (which were based on Russo-Japanese War) off. There is also strong arguments to be made that Naval forces from both side of the war didnt commit to full out pitched Naval battles in order to avoid decisive defeat that Russians faced twice during the Russo-Japanese War. Mahan had a big influence in Naval tactics throughout Russo-Japanese war, but because of Russo-Japanese war, navies began developing new ideas to counter and render Mahan's theories useless (like U-boat warfare).