Tear gas was developed by France and used on the battle field in WWI before being used for "crowd control".
https://www.kumc.edu/school-of-medicine/academics/department....
"Riot control agents, including tear gas and other gases which have debilitating but non-permanent effects as a means of warfare, is prohibited in armed conflict under the 1993 Chemical Weapons Convention."
https://ihl-databases.icrc.org/customary-ihl/eng/docs/v2_rul...
And, if a government's leaders feel they need to gas, assault with sound cannons / water cannons / pepper spray, use flash bang grenades, beat with clubs, and/or fire rubber coated steel bullets at their population, it might be taken as a hint that the government involved has lost the consent of the governed.
