Explanation: In WW1, one of the 'innovations' of the war was the widespread use of chemical weapons. One of the most terrible of them was Mustard Gas - named as such for its yellowish color and the smell. It is not nearly as delicious as it sounds - the gas causes horrific chemical burns after exposure to skin or inhalation.
After the war, the Geneva Protocol was signed, wherein signatories agreed that they would not use chemical weapons in warfare - with an exception carved out for retaliatory use. Despite the intensity of the conflict, and some nervousness over the matter, this prohibition held even during WW2 between the major belligerents, as both sides feared that the other would benefit more from the restriction being lifted.
It should be noted that the Japanese used chemical warfare on Chinese troops, who could not retaliate in kind for lack of such weapons, but not against American troops, whose government had the capacity to deploy chemical weapons if so provoked. Likewise, Nazi Germany executed over a million civilians using chemical gas, but refrained from using it on the battlefield for fear of retaliation by the Western Allies and Soviets.