22 April 1915. Second battle of Ypres starts. The Germans use chlorine gas against the French, Algerians and Canadians.
It works -sort of- until it doesn't.
And the winning is enough to drive development. Drive? Turbo-overdrive. The real chemistry war starts, to where by 1917 they would have three or more
tiered gas attacks. Imagine a heavier-than-air settling gas to get troops out of bunkers, a mucuous membrane and skin-attack itcher to get them to abandon protective clothing and gear, and a nerve agent for the actual kill. Unlike at Ypres, these would be delivered by fuzed artillery shells. Of course, you also need to develop artillery fuzes
French artillery doctrine (heavy on 75mm field guns, very low on howitzers) and the fact that most of their chemical industry was in or near the war zone means they never really catch up. The Brits and Germans have neither of these problems.