I just bought a recently released book, "Hitler's Great Gamble" by James Ellman. I'm currently in the Sea Lion scenarios portion of the book, and in it the author states that Churchill was planning on using different poison gases on the Germans should they have landed on the beaches. Is this common knowledge? He also mentioned that Hitler knew of this and could have threatened bombings of poison gas on British cities for retaliation. If Hitler knew the British were willing to do this, why didn't Hitler use poison gas against Allied invasions?