Wars are engagements whereby enemies inflict as much damage as possible on one another. Back then if a country had a weapon they used it without thinking. As time progressed so did weaponry all the way up to nuclear devices which once used caused countries to reexamine sanity to some degree and make promises to one another not to use mass destruction as a means of settling inane religious or economic disputes in combat. Now they employ computers to do their bidding. By the way, these wise leaders still stockpile poisons and big bombs just in case, but itchy fingers still exist.
By the end of WW1 in 1918, all the major powers including Russia had used chemical weapons. The British Empire as well as the German empire had also started using chemical weapons. The resultant gas attacks were later banned after the 1925 Geneva Protocol.