he didn't "forget" the holocaust. the holocaust has nothing to do with chemical weapons. the architect of gallipoli of course wanted to use poison gas in wwii, but none did because it would have been known that the allies were the first to employ it. the allies did have it accidentally released when one of their ships carrying mustard gas was bombed, and it was kept secret for decades that 96 americans mostly on shore near where the ship blew up in italy had died from american mustard gas. the numbers were higher than they should have been because the doctors didn't know it was mustard gas.
so the allies never did it because "hitler never used it". this is what we all know. i did mnot realize that the germans employed it in the crimea. they did it to smoke people out of caves