Why did Hitler not order the use of poison gas in combat?

I'm in possession of original WW2 British documents concerning chemical warfare. Those papers mainly concern experiments conducted in Hendon, on decontamination, detection and so on. It appears that the British were quite unaware of German nerve gas. A secret note does mention captured German ammunition describing something that could have been Tabun. The chemical was not being identified, but described as a probable mustard like only more potent and affecting the lungs. I don't know if anyone would be interested with those documents but I'd be willing to send them to anyone with academic interests in them

My father told me that it was asked of Churchill if it was possible that the Nazis might use some form of what we call chemical warfare today. Churchill apparently responded, "Let them try!" and this was sufficient of a deterrent to give Hitler pause, and he didn't use this tactic.

Using poison gas in warfare requires the user to wear a gas mask and a special suit. The Germans used blitzkrieg in WW2. The chemical suit would have slowed them up more than the allies.

An article from the NY Times[1] adds some insight here. It basically says "no one knows"! Contrary to some of the posts in this thread, the article cites military sources saying that use of nerve gas could have been a very effective battlefield weapon in WWII. The article mentions Hitler's personal experience with gas during WWI, fear of retaliation, and lack of delivery mechanisms as possible factors. It also cites another possibility: deliberate misinformation from Hitler's own general in charge of chemical use, who had become disenchanted with Hitler's leadership. But the bottom line: It's still a mystery!

That's all I got! I suppose we should all thank Sean Spicer for resurrecting this question ;-)

[1]: Riddle of Why Hitler Didn’t Use Sarin Gas Remains Unsolved https://www.nytimes.com/2017/04/12/science/sean-spicer-hitler-sarin-chemical-weapons-world-war-ii.html?_r=0

England and the Soviet Union had huge stockpiles of chemical weapons. There was a joint statement from Stalin and Churchill essentially confirming that they would observe the convention prohibiting them, unless Germany used them first. If Germany did use them, they promised to drown its cities in iprite. :-) The Soviets did not have the technical capability to do this in the early stages, but England did.

This information is taken from collected correspondence between Stalin and Churchill during World War 2, which I have in Russian. I can search for the date of the letter mentioning this joint statement if anyone is interested. I do not know whether this correspondence was ever published in English.

EDIT. There can be a completely different reason as well: chemical weapons are not very effective on the battlefield. The example of WWI shows this. Perhaps this explains why they were prohibited: nobody cared much about them.

In World War I all nations learned that poison gas basically added nothing to military actions, not even higher casualties once everyone got out their masks and protective gear. Artillery did better at inflicting damage just shooting high explosive shells.

In between the wars use of gas was banned, and most nations complied.

In World War II, every nation had plenty of stocks of gas to retaliate with and protective gear to issue if the other side started using banned weapons. This would make starting the use of gas a momentary advantage only. Also long range bombers meant that retaliation might strike deep to rear area installations or even homeland cities. So risks were much higher and the possible reward still close to zero.

There was an incident at Bari where some US gas stocks were released when a German raider attacked the ship and docks carrying it into Italy, for use if the protocols broke down.

In 1989 Rudibert Kunz (a journalist known for his work about chemical warfare) and military historian Rolf-Dieter Müller published an article about this question in the german newspaper Die Zeit (the article is available online). I found this via Google and was a bit surprised.

Like many people I had believed in the story that Hitler did not want to use gas because of his own trauma in the first world war (he was injured in a gas attack and temporarily blinded). It seems that this what not the case and that in fact Germany had produced large quantities of gas to be used to some extent against Great Britain, but mostly on the eastern front.

According to the article the use of gas against Britain was hampered by the fact that by the time chemical weapons where available in large quantities there was no way to deliver them (the A4 rockets had not enough payload for a large scale attack and planes could by that time not easily operate above british soil).

But apparently what really stopped the german plans was the fact that the allied forces had upped the ante by developing biological weapons which where (supposedly at least) much more deadly than mere poison gas. Germany had no big programs to develop biological weapons since Hitler did not believe they had enough potential to be useful (as I understand the article this had no deeper reason and was simply a misjudgment on his part. Apparently some Nazis underlings had run small developement progamms in concentration camps but (rather luckily) could not convince Hitler to allow large scale production). By the time the full potential/danger of biological weapons was recognized it was too late to catch up.

So the authors conclude that the Nazis did not use chemical warfare because the allies could issue a plausible threat to retaliate with much more deadly and uncontainable biologicals weapons (with the aim to wipe out lifestock and crops so that , by the quaint logic of some allied generals, they did not violate the Geneva protocol which protected human life only).

(edited to add) I realize that this does not quite answer your question, but it's the only thing on the topic I could find that was written by actual experts. And the article gives the impression that Hitler was not in a position to order a gas attack at the end of the war since none of his own military staff would have supported him (presumably even an absolute dictator needs someone to carry out his orders).

I'm not expert on this and just retell what I have just read. And of course the Nazis used a lot of poison gas, just not on the battlefield.

Re: Pierstorff's explanation, I haven't heard the biological weapons explanation before. As the primary motivation for Hitler's restraint, I don't think it's correct. From my understanding, bio weapons were rather primitive on all sides and the Japanese were actually the most advanced in this regard, not the Allies.

[EDIT: I'm not sure where I first heard all of this, but here's a confirming source that I do trust--Derek Lowe of In the Pipeline ]

I'm really surprised no one has mentioned tabun yet. This nerve agent was the first ever produced, the forerunner of sarin and VX. Hitler made a lot of it, and the Allies weren't even aware of nerve gases until they captured some tabun from the Germans fairly late in the war. The Germans could have wreaked real havoc if they had used it at a critical juncture. With an effective delivery system, they might have been able to turn back Operation Overlord (D-Day), prolonging the war for god knows how long. (Nerve gas tends to be much more effective than other gases, especially if the enemy doesn't know what the hell it is.)

Here's the punch line: the reason why Hitler didn't use any gas (tabun included) was because his advisers thought the Allies had tabun as well. Apparently all of the prerequisite chemical work had been done by American scientists and were published in ordinary academic journals years before (the discovery of tabun was a natural consequence of pesticide R&D.) They just assumed that the Allied military had taken note of the research and prevented further publication in order to use it as a secret weapon if Germany used gas first. In reality, Allied military scientists had completely overlooked it.

So, fearing the Allies' hefty air power could saturate German cities with tabun at a moment's notice, Hitler refrained from using any gas. I think this is a fascinating example of MAD functioning many years before the phrase entered common usage re: nuclear war... with the added twist that the threat from the other side was entirely imaginary.

It also seems to be an unusual example of Hitler showing fear, sensibility, and/or restraint.