The history is even more complicated than that. After the great loss of life at Saipan - I believe it was 900 US dead a day - the "Joint Chiefs" wanted to use poison gas on the Japanese. FDR said no but his only reason was that he was afraid the Japanese would retailiate. (I'm sorry I can't source this but I read it a long time ago and didn't save the reference).
That is, would retaliate by using poison gas against our troops.