They apparently used it in the Crimea. (Some sources say Sevastopol, others Kerch.)
According to Wikipedia, when they interrogated Goering after the war, he told them the reason they didn't use their nerve gas to repulse the landings at Normandy was that they hadn't been able to make an effective gas mask for horses. The german army still relied primarily on horses for transport, and everyone learned in WWI that gas doesn't always go where you want it to.
At the end, Hitler didn't care a fig what happened to Germany. He said they had failed their destiny, and he ordered destruction of their own infrastructure. He also dragged the war on for months after it was obviously lost, to the great harm of the Geman people.
If deterrence worked, we wouldn't have had two world wars.