Trial of the Major War Criminals before the International Military Tribunal. Volume XVI. Proceedings June 21, 1946. pp. 524-528.

Note that the gases Speer refers to here are extremely fast-acting and deadly. Compare these descriptions to the known physical properties of Zyklon-B, the insecticide said to have been used by the Third Reich to murder millions of Jews and others. Then ask yourself, why would the Germans have used a slow-acting insecticide rather than a fast-acting poison gas, of which they had huge stockpiles?

[…]

MR. JUSTICE JACKSON: Leaving the question of your personal participation in these matters and coming to the questions dealt with in the second part of your examination, I want to ask you about your testimony concerning the proposal to denounce the Geneva Convention.

You testified yesterday that it was proposed to withdraw from the Geneva Convention. Will you tell us who made those proposals?

SPEER: This proposal, as I already testified yesterday, came from Dr. Goebbels. It was made after the air attack on Dresden, but before this, from the autumn of 1944 on, Goebbels and Ley had often talked about intensifying the war effort in every possible way, so that I had the impression that Goebbels was using the attack on Dresden and the excitement it created merely as an excuse to renounce the Geneva Convention.

MR. JUSTICE JACKSON: Now, was the proposal made at that time to resort to poison gas warfare?

SPEER: I was not able to make out from my own direct observations whether gas warfare was to be started, but I knew from various associates of Ley’s and Goebbels’ that they were discussing the question of using our two new combat gases, Tabun and Sarin. They believed that these gases would be of particular efficacy, and they did in fact produce the most frightful results. We made these observations as early as the autumn of 1944, when the situation had become critical and many people were seriously worried about it.

MR. JUSTICE JACKSON: Now, will you tell us about these two gases and about their production and their effects, their qualities, and the preparations that were made for gas warfare?

SPEER: I cannot tell you that in detail. I am not enough of an expert. All I know is that these two gases both had a quite extraordinary effect, and that there was no respirator, and no protection against them that we knew of. So the soldiers would have been unable to protect themselves against this gas in any way. For the manufacture of this gas we had about three factories, all of which were undamaged and which until November 1944 were working at full speed. When rumors reached us that gas might be used, I stopped its production in November 1944. I stopped it by the following means. I blocked the so-called preliminary production, that is, the chemical supplies for the making of gas, so that the gas production, as the Allied authorities themselves ascertained, after the end of December or the beginning of January, actually slowed down and finally came to a standstill. Beginning with a letter which is still in existence and which I wrote to Hitler in October 1944, I tried through legal methods to obtain his permission to have these gas factories stop their production. The reason I gave him was that on account of air raids the preliminary products, primarily cyanide, were needed urgently for other purposes. Hitler informed me that the gas production would have to continue whatever happened, but I gave instructions for the preliminary products not to be supplied any more.

MR. JUSTICE JACKSON: Can you identify others of the group that were advocating gas warfare?

SPEER: In military circles there was certainly no one in favor of gas warfare. All sensible Army people turned gas warfare down as being utterly insane since, in view of your superiority in the air, it would not be long before it would bring the most terrible catastrophe upon German cities, which were completely unprotected.

MR. JUSTICE JACKSON: The group that did advocate it, however, consisted of the political group around Hitler, didn’t it?

SPEER: A certain circle of political people, certainly very limited. It was mostly Ley, Goebbels and Bormann, always the same three, who by every possible means wanted to increase the war effort; and a man like Fegelein certainly belonged to a group like that too. Of Himmler I would not be too sure, for at that time Himmler was a little out of favor with Hitler because he allowed himself the luxury of directing an army group without being qualified.

MR. JUSTICE JACKSON: Now, one of these gases was the gas which you proposed to use on those who were proposing to use it on others, and I suppose your motive was …

SPEER: I must say quite frankly that my reason for these plans was the fear that under certain circumstances gas might be used, and the association of ideas in using it myself led me to make the whole plan.

MR. JUSTICE JACKSON: And your reasons, I take it, were the same as the military’s, that is to say, it was certain Germany would get the worst of it if Germany started that kind of warfare: That is what was worrying the military, wasn’t it?

SPEER: No, not only that. It was because at that stage of the war it was perfectly clear that under no circumstances should any international crimes be committed which could be held against the German people after they had lost the war. That was what decided the issue.