So, do we keep Fritz Haber's contribution?
If so, we need to do away with the bulk of the massacres in WW2 so that mass murder remains seen as a viable method of warfare, at least far from home.
Colonial empires come stronger out of the war and perhaps the UN is not formed at all. As you may recall, TTL LoN was somewhat reluctant in sanctioning the use of gas against Ethiopia, so perhaps it will happen again.
What's the use case though? Last resort against widespread rebellions or something more systematic? It's hard to imagine France and Britain doing the latter, though (and even less the Netherlands and Portugal).