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Why did most major powers avoid using chemical weapons in Europe during World War II?

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American soldiers in World War I uniforms, some wearing gas masks, advance with rifles during a simulated gas attack exercise on a rocky battlefield. — chemical weapon American soldiers participating in a simulated gas attack exercise, World War I.

During World War II, most major powers refrained from using chemical weapons primarily due to the fear of retaliation. Although the 1925 Geneva Protocol prohibited the use of poison gas and biological weapons in war, it did not ban the development, production, or stockpiling of these agents.

By the start of the conflict, most major powers had already developed robust chemical warfare capabilities. Because many signatory states had reserved the right to retaliate in kind if an adversary used chemical weapons first, the possession of these weapons by opposing sides created a state of mutual deterrence. Consequently, all major powers except for Japan were deterred from initiating chemical warfare, as they feared the devastating consequences of a reciprocal attack from their enemies.1