Poison gas was used in WWI with terrible effect. However, both sides developed sophisticated gas masks and protective clothing that essentially negated the strategic importance of chemical weapons.
After WWI some claimed that chemical weapons would be useful in successive wars.
Mustard gas was used by the Italians in their campaign in Abyssinia from 1935 to 1936.
At the start of WWII, both the Allies and Axis powers had vast stockpiles of poison gas – therefore, its use was a very real fear. In Britain, civilians were issued with gas masks and drills became a routine.
But chemical warfare never became a major issue in WWII. One theory why gas attacks were not deployed is that, by the 1940s, all warring nations had the technology necessary to produce various forms of poison gas. Since similar counter attacks would undoubtably follow an attack with gas, there was no tactical advantage to the use of this dreadful weapon.