During World War II, the German atomic energy program decided, by 1942, to be entirely focused on reactor development. This was, they thought, the area of nuclear fission research was most likely to produce useful military results in the short term. They thought bombs were probably possible but incredibly difficult to construct, and did not think any other nation was going to have any success with them in the short term. The German teams honestly thought they were ahead of the entire world in fission research, and when a group of the top people were carted off to a British manor house, Farm Hall, at the end of the war, they thought that maybe the Allies were trying to keep them from giving their knowledge to the Soviets (true) and that the Allies would want their knowledge for themselves (false).

When they heard about the bombing of Hiroshima, they were shocked and surprised. Many of them refused to believe the United States could have pulled off a bomb. After getting more news information and talking it over, they realized it was in fact possible if the USA had thrown tremendous resources at it, and also realized that some of their assumptions about the size of the critical mass for the bomb were off.

This left them in a tricky situation in terms of their egos and their nationalism. Were they simply failures? This is what Otto Hahn (discoverer of fission, and no fan of Hitler) suggested — that they were just second-raters.

But many of them started to create and cling to a different story. They weren't accidental failures — maybe they were deliberate failures. Maybe they didn't really want to make a bomb, and that's why they didn't do it. This isn't the same thing as saying they intentionally sabotaged the project, but it was more of a psychological explanation. If they had wanted to make a bomb, surely they would have done more work in that direction, right? So they must not have wanted to make a bomb. Because who would want to give Hitler a bomb?

This story — known to historians as the Lesart of the German atomic program — was propagated in subtle ways in the aftermath of World War II. It achieved its initial circulation in Robert Jungk's _ Brighter than a Thousand Suns_ (Heller als tausend Sonnen) in 1956. Jungk was the first to suggest, based on a vague Lesart explanation offered up by Heisenberg, that Heisenberg had intentionally sabotaged the project. Heisenberg himself was somewhat appalled by this. Here is what he wrote to Jungk in 1956:

You speak here towards the end of the second paragraph about active resistance to Hitler, and I believe—pardon my frankness—that this passage is determined by a total misunderstanding of a totalitarian dictatorship. In a dictatorship active resistance can only be practiced by people who seemingly take part in the system. When someone speaks openly against the system, he quite certainly deprives himself of any possibility of active resistance. For either he then expresses his criticism of the system only occasionally in a politically harmless form, and then his political influence can be blocked very easily; say, with respect to the youth, by people saying: Sure, old Professor X is a nice old man, but he naturally has no understanding for the enthusiasm of the youth, or something of the sort. Or, conversely, he really tries, say, to move the students politically, and then within a few days he naturally would meet his end in a concentration camp, and even his self-sacrificing death would remain practically unknown, since no one would be allowed to talk about it. I would not want this remark to be misunderstood as saying that I myself engaged in resistance to Hitler.

Which is rather equivocating to say the least. Heisenberg isn't actually implying he did anything in this particular letter, though he is implying that he did some kind of passive resistance.

Historians have gone over Heisenberg's wartime activities in detail. There's no evidence whatsoever of intentional sabotage. Heisenberg actually went to great lengths to show he was in line with Nazi Germany's overall foreign intentions, and used his position to stump for Germany and German culture in occupied countries. He vigorously pursued reactor research. He did not vigorously pursue bomb research, but this is because, again, he did not think it was feasible in the short term. Here is another Heisenberg letter to Jungk, from 1957:

[In 1941] we in the uranium project had come to the following conclusion, on the basis of our experiments with uranium and heavy water: It will definitely be possible to build an energy-producing reactor out of uranium and heavy water. In this reactor (on the basis of theoretical work by von Weizsäcker) a product of U-239 [U-239 decays into Plutonium-239] will be generated that, like U-235, will be suitable as an explosive for atomic bombs. At the time we did not know of any process that could have produced significant quantities of U-235 with an effort that could be realized technically in Germany under wartime conditions. Since the production of atomic explosives using reactors could also obviously be realized only through many years of running huge reactors, it was clear to us in any case that the production of atomic bombs would only be possible with an enormous technical effort. Thus we knew that atomic bombs could be made in principle, but we also estimated that the necessary technical effort was bigger than it really was.

This situation seemed to us a particularly favorable precondition for the physicists to have an influence on the further course of events. For if the production of atomic bombs had been impossible, then the problem would not have arisen at all; but if it had been easily possible, then the physicists surely would not have been able to hinder their production. But the actual situation gave the physicists at this time a decisive influence on the further course of events, since they could argue vis-a-vis their governments that the atomic bombs would probably not come into play in the course of the war, or else argue that it would instead perhaps still be possible, with absolutely enormous efforts, to put them into play. That both ways of arguing were objectively fully justified was shown by the course of developments; for in fact even the Americans could not put the atomic bomb to use against Germany.

The first paragraph of this letter is born out through documentation — the Germans knew that reactors might be a route to the bomb, but they also knew it would require large reactors. Their work on a small research reactor (which would have been worthless for bomb use) was slow and piecemeal and did not reflect any active bomb concern. They knew about uranium enrichment but considered it too difficult to really pursue in earnest.

The second paragraph is the more problematic one. Again, Heisenberg isn't actually saying that he sabotaged anything. What he's saying was, under the conditions that existed, physicists could have pushed for the bomb if they really thought they could do it, but nobody in the government would fault them if they didn't. Is he saying they didn't push for the bomb because they didn't want Hitler to have one? No. But he's allowing that interpretation to be made if you want to make it.

In short, even Heisenberg himself didn't ever really claim that he sabotaged the German atomic bomb project in any real way. At best he implied that maybe he wasn't as enthusiastic about the bomb as he could have been, which is in line with the preservation of the idea that he wasn't just a screw-up on this front. But there's no evidence he did anything like that. Of course, as Heisenberg himself pointed out in the first letter I quoted, the nature of being a passive resister in a totalitarian government is that you appear to be going along with the plan but you might not actually be going along with it. This makes the Heisenberg issue a tricky one for historians, because, as he points out, if he simply withheld enthusiasm for the bomb but otherwise made like a good German, it would be indistinguishable from the situation where he just didn't push for it because he thought it wasn't very feasible.

I think the Farm Hall transcripts make it clear that Heisenberg truly didn't think it was feasible, and in fact hadn't thought seriously about the physics of atomic bombs at all. (His discussions there reflect deep misunderstandings about how fast neutron fission chain reactions work.) Which to me puts him in the "was just a screw-up" camp, as opposed to the "was silently and passively resisting" camp.