The Allies, and the Germans, did not engage in chemical warfare that I know of. There were stockpiles held in reserve in case the other side unleashed theirs, but the memories of the mustard gas and phosgene (phosphorus?) gasses in the trenches of WW1 was too fresh in everybody's minds, plus even Hitler realized that, with the advent of modern weapons such as long-range artillery and aircraft, once the covers were yanked off chemical warfare, then no city was safe. A flight of Lanc's or -17's over Berlin loaded with mustard gas didn't have to actually have a specific target in mind, just unloading on a city would be catastrophic to the civilian populations, which would seriously damage production capabilities until facilities could be set up in "safer" locations. The only use of chemical warfare (that I recall, anyway) being unleashed was in Italy, where a transport ship with a bunch of mustard gas in the holds (Eisenhower had it shipped over to be held in ready reserve since there were strong rumors that Hitler/Mussolini were about to start using their own gasses....which just sounds wrong, and I can't help but snicker when I re-read that) was hit with a random bomb from a German air raid. The ship exploded, spreading mustard gas all over the harbor. Troops/seamen pulled out of the harbor with nothing more than a thorough soaking were soon crowding the hospitals. Rescuers were likewise soon overcome, yet because of the "gentleman's agreement" that no gas warfare was going to take place, medics were in the dark that they were actually dealing with mustard gas burns/symptoms, and lots of Allied troops died due to the lack of communication. I think it was finally whispered to the medics that "those symptoms look suspiciously like mustard gas, oh, hey, the treatments for that actually work, lets just go with it and not ask any questions". Eisenhower hushed the incident up, not wanting to be the first commander to be caught with gas in his front line storage areas, as well as being guilt-ridden over the loss of lives caused by his decision to move it closer to the front.