>>509999239
Chemical warfare started in WW1 when the French started using teargas (ethyl bromoacetate) against the Germans.
https://en.wikipedia.org/wiki/Chemical_weapons_in_World_War_I

The issue boils down to: Is there really a difference between using teargas to force someone out of a trench/bunker/tunnel so you can shoot them, or just using a poison gas to kill them in the first place.

Police use of teargas typically isn't intended to facilitate the subsequent killing of the people who are gassed.