While the Americans were pursuing an atomic bomb during World War II via the Manhattan Project, Nazi Germany had a competing effort of its own dubbed the Uranium Club. The project, led by nuclear physicist Kurt Deibner, and the Allies’ high-stakes mission to foil it, take center stage in Neal Bascomb’s new book The Winter Fortress: The Epic Mission to Sabotage Hitler’s Superbomb.

The German bomb design relied on the rare substance heavy water, which has an atypically high amount of the hydrogen isotope deuterium in it, to slow down the neutrons released by nuclear fission in order to create a chain reaction. The Nazi effort got a boost when Germany occupied Norway, which hosted a hydroelectric plant called Vemork that was the world’s only supplier of this substance. Yet when Germany invaded, Vemork’s chemist Leif Tronstad, who had invented the technique to synthesize heavy water, became an intelligence agent for the Allies—thus embarking on a mission to help destroy his own life’s work.

Scientific American spoke to Bascomb about this World War II tale of science and spy craft.

[An edited transcript of the interview follows.]

Norwegian chemist and professor Leif Tronstad is the core character of your book. He was also a major player in heavy water research. What got Tronstad interested in heavy water in the first place?
Tronstad was a very interesting guy. He was a professor, but he was also a consultant to a number of industrial firms and he loved original research and experimental physics. For him, he didn’t care what the economic potential [of heavy water] was. He thought it may be useful in nuclear research—and when I say nuclear, I mean artificial radioactivity and the like, not necessarily splitting the atom—or thought it might be useful as a good way to make an ice skating rink because it would freeze at a better temperature. He didn’t know, is the short answer, but it was an interesting academic experiment. When the atom was split for the first time, suddenly heavy water as a moderator for nuclear chain reactions was something that everybody wanted.

So Tronstad figures out to make heavy water and then becomes one of the central figures in destroying his own creation. How did that happen?
Here’s a man who built this plant—this amazing feat of engineering and science that secured his name in history—and then for the next four years his mission is to destroy it. That’s interesting, if nothing else. He’s the scientist as hero in every respect of the word. Tronstad had no military training, no intelligence training. The Germans occupy, he starts a spy network informing the British what’s going on and he’s targeted by the Gestapo for that. He escapes, leaves his family behind, and gets to England. The British scientific establishment says that he should join them and he says: No, I want to fight as actively as possible for the freedom of Norway.

Within the span of two to three months you have Leif Tronstad the chemistry professor suddenly at the nexus of British special services and Norwegian high army command. He’s playing the role of spymaster, operational leader, and using his awareness of all Norwegian industrial firms and what they produce to inform what needed to be attacked. The top of that list was Vemork.

Shifting from scientist to spy is a pretty uncommon career move. Did Tronstad’s background help him as an intelligence agent?
It was because he was a scientist, the way that he thought and broke down a problem, the way he tested things—he designed the operation not necessarily like a spymaster but as a scientist would. His thought process was to ask what’s effective, what’s not effective, let’s test this, and then come up with the exact right way to do this.

The book chronicles this epic espionage mission but the German atomic bomb team plays a strong secondary role. What stands out to you about that team?
[Werner] Heisenberg is an individual that everybody talks about when they talk about the atomic bomb. But as I researched this story, I found that he wasn’t the essential player. Deibner was the one who started the German Uranium Club, he was the one who launched its research and unarguably was the one pushing the most for Germans to obtain the bomb. Heisenberg was not actively attempting to build the bomb for the Germans—that was Deibner.

You’re a historian, not a scientist. How did you tackle explaining the science at the heart of the espionage?
I read an enormous amount in order to write these things, so I’ve read a lot of books on atomic science and heavy water. The hard part of writing this book was writing about the science of heavy water, so at the end of the day I decided to stop myself and write as if I was explaining it to a sixth grader. And then I had some very smart people, like heads of atomic programs and engineers who build nuclear power plants, read it and tell me where I was wrong and help me rewrite it. It was, in every sense of the word, a team effort. I had to write the history of atomic science in 600 words.

At the core of things, what interests me about history are people and what drives them, what makes them successful, what makes them unsuccessful, how do they go about achieving or not achieving what they want to do. And I think there’s something pure about what scientists are trying to achieve— for no other reason than to see if it can be done. It just makes for great history. I love reading about scientists, about people experimenting and trying to do things. I think it’s a window into the human soul.