La Vie Quotidienne des Français au XXe Siecle. 1900-2000. Booster-LPM, 1999.
I read that, during the Nuremberg trials, Goering was asked by an American interrogator why the Germans hadn't used gas during the Normandy invasion. He replied, "Die Pferde!" ("The horses!") He explained that the German armed forces at that point in time still relied heavily on horse-drawn transport and no one had ever been able to develop a gas mask that worked well on the animals. The Germans expected that, if they used gas, the Americans would retaliate in kind. The loss in horses would have collapsed the German war effort very quickly.