Title. I am well aware of the extensive usage of chemical and biological weapons by the Japanese in China, but am curious that, aside from a few half-baked schemes that never went forward [Operation Cherry Blossoms At Night], I am not aware of any incident in which the Japanese targeted 'Western' allied forces with chemical weapons, or even, say, Filipino guerrillas. I'm more than a little surprised by this 'fact', as Japan showed that it had no compunctions about using WMDs and viewed the Geneva Convention as more of a Geneva Suggestion.

I have a few theories:

1. Chemical weapons were Kwantung Army doing Kwantung Army stuff. Their chemical weapons programme was basically independent from the Japanese military as a whole and especially the IJN.

2. Japan never used chemical weapons against the US or allies knowing that [unlike the Chinese whom had no chemical capabilities and thus could not respond in kind] they would have zero qualms of retaliating in greater quantity, something which would especially tax, for instance, remote and isolated island outposts riddled with bunkers.

3. Logistical constraints meant that bringing chemical weapons over from Manchuria or even the home islands was viewed as either impractical or wasteful.

4. The Japanese did use chemical weapons on allied forces and I just never heard of it.

However, if someone here could more thoroughly enlighten me on this, I would most appreciate it.

Edited slightly for clarity.