Because chemical weapons are a qualitatively different to a bullet or missile. They're inaccurate, indiscriminate weapons that affect large areas, so chemicals can't distinguish between armed combatants and civilians. And to be gassed is a horrible, agonising way to die compared to simply being blown up or shot. GAS! Gas! Quick, boys! – An ecstasy of fumbling,Fitting the clumsy helmets just in time;But someone still was yelling out and stumbling,And flound'ring like a man in fire or lime . . .Dim, through the misty panes and thick green light,As under a green sea, I saw him drowning. In all my dreams, before my helpless sight,He plunges at me, guttering, choking, drowning. If in some smothering dreams you too could paceBehind the wagon that we flung him in,And watch the white eyes writhing in his face,His hanging face, like a devil's sick of sin;If you could hear, at every jolt, the bloodCome gargling from the froth-corrupted lungs,Obscene as cancer, bitter as the cudOf vile, incurable sores on innocent tongues,My friend, you would not tell with such high zestTo children ardent for some desperate glory,The old Lie; Dulce et Decorum estPro patria mori.