Hi!
I'm assuming this has been done before, but here goes.
It's 1944 or so. The Germans are being pushed back on all fronts and find themselves in increasingly desperate straits. They're concerned that they'll have to do something big to stave off defeat.
While the war has been going on, German scientists have been working on developing increasingly deadly gases to use on the victims of the death camps. Someone in the General Staff figures that the Allies have not learned about the particulars of these new gases and thinks that they might give the Germans an advantage on the battlefield -- especially if one of the gases is such that the WWI-era gas masks will prove to be of no protection to its victims.
Hitler is initially opposed to the idea: he believes the Final Solution has to keep going, and he is reluctant to unleash gas on the battlefield after suffering a gas attack himself in WWI. However, the General Staff talks him into it. Germany has to do something radically different to change the tide of battle. Furthermore, word of the concentration camps is starting to leak out and other countries are starting to complain to the Nazis about them. The officer recommends that the camps be scaled back. This leaves all the gas stockpiled: might as well do something with it.
If Hitler's dead set on the concentration camps, arrangements could be made to test the gas on camp victims wearing gas masks until they can find a gas which can affect a person with a gas mask (this sounds sort of like Mengele's "experiments" with the camp victims). If the Germans are lucky, these experiments could result in a gas which will defeat WWI gas masks but not newly-invented Nazi-made gas masks (this can be discovered by a victim in the camps surviving the gassing with the experimental mask, at which point the victim is shot and the mask is deemed a success)
So, the Germans decide to use gas. The gas canisters could be shot by mortars or sprayed from aircraft as an aerosol (provided that the Germans can protect their own troops). Gas bombs could also be used.
Presumably, the Allies would respond in kind -- but the Germans would probably figure their new gases will be more effective than the old ones.
How would the war continue from this point?
ACG