September 1941 was a high water mark for Nazi Germany. Though defiant, Great Britain was reeling. The Soviet Union appeared on the brink of collapse. Against this background a pivotal event occurred in occupied Denmark, one with enormous impact on the atomic bomb programs of Nazi Germany and the Allies: the meeting between Niels Bohr and Werner Heisenberg.

Both were Nobel laureates who received their awards in physics. Bohr was regarded as one of the most influential scientists of the twentieth century, not only for his research but also because he taught and mentored so many other scientists. His brightest pupil was Heisenberg, whom the Dane treated like a son.

“It was from September 1941 that we saw an open road ahead of us, leading to the atomic bomb.”

– Werner Heisenberg

When Adolf Hitler became chancellor in 1933, like it or not Germany’s scientists had to make a political decision: support the new Nazi regime, or (if possible) leave the country.

In the years leading up to World War II’s outbreak, Nazi Germany did suffer a brain drain as many scientists, most of them Jewish, left. Those that remained broadly fell into two groups, those that took the “ivory tower” approach and tried to shut out Nazi interference as much as possible, and those who, for various reasons, actively worked for the Nazis. Of the latter, some embraced the regime while others did so more reluctantly, viewing it as a Faustian bargain with a Nazi devil. The most notable example of the former was the rocket scientist Werner von Braun, of the latter was Werner Heisenberg, the head of Nazi Germany’s atomic bomb program, nicknamed the Uranverein, or Uranium Club.

On Sept. 15, 1941, Heisenberg arrived in Copenhagen, ostensibly to participate in a symposium on astronomy, mathematics, and theoretical physics at the new German Cultural Institute. But his real purpose was to visit Bohr, his mentor, which he did three times.

It is Heisenberg’s second visit to the Bohr residence that has since become the center of historical interest and controversy, as that was the one in which Heisenberg brought up the subject of the atomic bomb.

The first meeting went well, with the two enjoying the renewal of their friendship. But during the period between the first and second meetings, when Heisenberg attended the symposium, Heisenberg made a critical misstep. He undiplomatically attempted to convince his colleagues that they should side with Germany, because it would soon win the war.

When Heisenberg arrived for his second meeting, the atmosphere was chilly. Then when Heisenberg got down to business, talking about the atomic bomb, things rapidly deteriorated. Later that day, summarizing the meeting to a colleague, Heinsenberg said, “You know, I’m afraid it went badly wrong.” The third meeting, another social event, was pleasant and the two parted amicably.

But their relationship had irrevocably changed; they never saw each other again. In late 1943, warned that the Gestapo were planning to arrest him, Bohr escaped to America to assist the Allies’ atomic bomb program.

Neither made a record at the time of the second meeting’s conversation, giving rise to speculation on what exactly was discussed, and why. Did Heisenberg hope to convince Bohr that Nazi atomic bomb progress was more propaganda than fact – that Heisenberg and his colleagues were actually sabotaging development? Did he hope to enlist Bohr – a Jew – in the Nazi atomic bomb effort?

Heisenberg himself, in a letter to his wife Elisabeth, hinted that he and Bohr were in disagreement politically:

Neither made a record at the time of the second meeting’s conversation, giving rise to speculation on what exactly was discussed, and why.

“The conversation quickly turned to the human concerns and unhappy events of these times; about the human affairs the consensus is a given; in questions of politics I find it difficult that even a great man like Bohr can not separate out thinking, feeling, and hating entirely. But probably one ought not to separate these ever.”

Some, like Heisenberg’s Uranverein colleague Peter Jensen, believed that Heisenberg, “the high priest of German theoretical physics, sought absolution from his Pope.”

After the war, Heisenberg claimed he was secretly preventing the Nazis from building an atomic bomb and was attempting through Bohr to get nuclear scientists to halt nuclear weapon development. Bohr’s response, made public years after his death, offers a different, less noble, version.

A Danish edition of a book covering the early atomic scientists and development of the atomic bomb included excerpts of a letter from Heisenberg detailing his version of the meeting. Bohr wrote, but never sent, a letter to Heisenberg, making clear his memory of events, which was among documents surrounding the affair that were released to the public in 2002.

“…I think that I owe it to you to tell you that I am greatly amazed to see how much your memory has deceived you in your letter to the author of the book, excerpts of which are printed in the Danish edition,” Bohr wrote.

“Personally, I remember every word of our conversations, which took place on a background of extreme sorrow and tension for us here in Denmark. In particular, it made a strong impression both on Margrethe and me, and on everyone at the Institute that the two of you spoke to, that you and Weizsäcker expressed your definite conviction that Germany would win and that it was therefore quite foolish for us to maintain the hope of a different outcome of the war and to be reticent as regards all German offers of cooperation. I also remember quite clearly our conversation in my room at the Institute, where in vague terms you spoke in a manner that could only give me the firm impression that, under your leadership, everything was being done in Germany to develop atomic weapons and that you said that there was no need to talk about details since you were completely familiar with them and had spent the past two years working more or less exclusively on such preparations. I listened to this without speaking since [a] great matter for mankind was at issue in which, despite our personal friendship, we had to be regarded as representatives of two sides engaged in mortal combat.”

Regardless, it was fortunate, if that’s the right word, that of the two Werners, Werner Heisenberg, a poor administrator, was in charge of Uranverein and not Werner von Braun. Had their roles been reversed it’s not too great a stretch of the imagination to believe that von Braun, as brilliant and ruthless an administrator as he was a scientist, would have given Nazi Germany the world’s first atomic bomb instead of the V-2 rocket.