Everybody wins.
It seems that a non-lethal / no permanent disability hallucinogenic agent would not violate the treaty against chemical/biological weapons, would basically achieve passive area denial, with create zero casualties, and would (if it actually worked as intended) probably be a heck of a lot of fun (ignoring all the un-fun aspects of war like "these new people now own this territory and here are the consequences for everyone who used to own it".)
Note: if a gas is impractical as delivery mechanism (hello, gas masks), be creative: a 'contact agent' (skin absorption ~~ like DMSO), cheap 'cluster darts' sprayed from drones, etc, etc.