On February 27, 1942, nine saboteurs scaled a cliff in the middle of the night to blow up a Nazi-controlled heavy water plant in Norway. Hollywood turned the story of the attack into The Heroes of Telemark, a sappy action-movie-on-skis starring Kirk Douglas. The true story is both more complicated—and more compelling. Using rarely viewed Norwegian records, eyewitness accounts, and his own travels in Norway, Neal Bascomb's The Winter Fortress: The Epic Mission To Sabotage Hitler’s Atomic Bomb sets this daring sabotage mission in the context of the high-stakes race between the Germans and the Allies to create a nuclear weapon.

Speaking from his home in Seattle, Bascomb explains how geography made the Vemork plant so hard to attack; how two of the greatest German physicists of the age, Einstein and Heisenberg, ended up on opposite sides of the race to create a nuclear bomb; and why the Norwegian commandos who blew up the plant were ordinary men, who did extraordinary things.

View Images Courtesy of Houghton Mifflin Harcourt

You write that Allied leaders in World War II regarded the heavy water plant at Vemork as “on the thin line separating victory and defeat.” Put it on the map for us—and explain why it was so important.

Vemork is about 100 miles west of Oslo, on the edge of this ice-bound precipice. It was the only plant in the world that produced heavy water, which was the key ingredient in the German atomic bomb research program. They needed heavy water to create a nuclear reactor, which was the stepping-stone to producing plutonium, and then an atomic bomb. The Allies did not know how far along the Germans were [in their researches] but the one thing they did know was the Nazi concentration on heavy water. So they took that one spot, and hit it.

Winston Churchill called heavy water “a sinister term, eerie, unnatural.” Deconstruct it for us.

Hydrogen has a single proton and a single electron. Deuterium, which is an isotope of hydrogen, has a neutron in its nucleus. This makes it heavier. It is very rare—there is one part heavy water for every 41 million molecules of regular water— and has this ability as a so-called “moderator.” In an atomic reaction, it slows down bombarding neutrons and doesn’t absorb them, which fosters a chain reaction.

To produce heavy water you need a lot of water and a lot of power. And Vemork had both of those. It also had this ingenious production facility, invented by a man named Leif Tronstad, a Norwegian professor, who thought, Here we have a lot of water and a lot of power, so we can create heavy water, which we hope the world will need.

At the outset of WWII, a race began to create a nuclear weapon. Give us an overview of the participants.

Nuclear fission was discovered in 1939. Very quickly after the start of WWII, the Allies were thinking, We can harness nuclear fission to do one of two things: create power or make a bomb. Einstein and a group of physicists who’d escaped Germany began research as early as 1940, assembling supplies and trying to figure out whether an atomic bomb was feasible. The exact same thing was happening in Germany. A German physicist named Kurt Diebner was the spearhead of the program. He recruited a number of physicists, among them Nobel Prize-winner Werner Heisenberg, who began doing the basic research, getting heavy water and figuring out, like the Allies, how to create a reactor and a bomb. By 1942, they were all, in essence, in the same place.

The British team working on the bomb had a wonderfully Orwellian name, the Directorate of Tube Alloys. Give us a peek behind closed doors.

The British were investigating nuclear energy at the same time as the Americans, on a separate but very much dual track. They did a lot of the early research to prove that an atomic bomb was feasible, and in 1942 they joined forces with the Americans, thanks to Roosevelt and Churchill. The Directorate of Tube Alloys was one of those curious code names the British love. [Laughs.]

View Images Leif Tronstad (seated, center) came up with the plan to take down the heavy water plant he had designed. He is shown here with members of the Gunnerside mission. Photograph by Norges Hjemmefrontmuseum

The hero of your book is a Norwegian whose code name was The Mailman. Tell us about Leif Tronstad.

Tronstad was a 38-year-old Norwegian professor of chemistry at the University of Trondheim, who came up with the idea of the heavy water plant at Vemork. In 1940, the Germans invade and take over Vemork. Tronstad fights in the battle for Norway, which the Norwegians lose, and goes back to teaching in Trondheim. Simultaneously, he begins working for the underground, providing the Allies with intelligence on the German interest in heavy water.

Within months, he realizes he has to escape Norway, so he leaves his family behind and goes to England. What’s so interesting about Leif Tronstad is that he’s a brilliant scientist with a brilliant mind, but all he wants to do is be parachuted back into Norway to fight on the ground against Germans.

The obvious question is why didn’t they just bomb the plant back into the Stone Age?

[Laughs.] That’s what the Americans wanted to do in early 1942! The British were also willing to do that. It was Tronstad who said, No, you’re going to kill a lot of civilians if you do that and you’re not gonna hit it. The key part of the heavy water plant was in the basement of this enormous fortress of steel and stone. You can bomb it night and day, but you’re probably still not going to hit the essential parts of it.

View Images The head of German forces in Norway, General Falkenhorst, called the attack on Vemork "a brilliant coup." Today, the plant serves as the Norwegian Industrial Workers Museum. Photograph by David Gale, Alamy

The Norwegian saboteurs were trained by a top secret British unit called Special Operations Executive (SOE). Tell us about this famous organization—and the "merciless regimen" of their training in Scotland.

The SOE was known as “the ministry of ungentlemanly warfare.” They were saboteurs, dropping behind enemy lines to hit the Nazis in guerrilla attacks on industrial plants and railway lines. The Norwegian branch of SOE, Company Linge, recruited young Norwegians who had either taken a boat or flown to England to be trained.

They recruited the best of the best. Then they put them through this intensive training regimen. A lot of the training was done in Scotland. They would go on night training exercises in the mountains, fording rivers, crossing passes, and sleeping outside for weeks on end. On the mental level, they learned how to handle strain and stress. I don’t think it’s too dissimilar to Special Forces training today.

Vemork is incredibly remote. How did the geography and conditions affect the sabotage attempt?

Vemork is a natural fortress. It could only be reached by a single-lane suspension bridge and was surrounded by an area called the Hardangervidda, a high mountain plateau where, according to legend, it grows so cold, so fast, that it freezes flames in the fire.

There were also minefields, searchlights, high barbed wire fences, and constant patrols. The saboteurs parachuted into the area in October, to prepare for the mission and collect intelligence. They had to survive on their own in wintertime, with temperatures in negative Fahrenheit, on this barren mountain plateau, which is basically rock, snow, and wind. They have very little food, so they have to hunt reindeer, and there’s no way to travel except on skis. Thankfully, they were Norwegians, so they were born with skis on their feet. [Laughs.]

After months of preparation, the team finally reached their objective. Put us on the ground on the night they blow up the plant. First, they’ve got to climb this bloody mountain, haven’t they?

On the night of February 27-28, 1942, everything is ready for Operation Gunnerside. Nine of the saboteurs are led by a 23-year-old named Joachim Ronneberg, who had no military experience except for what he had learned in England. The first thing they have to figure out is how to attack the place. They have three options: they can come down from the mountains above the plant, which is littered with minefields; they can cross the single-lane suspension bridge, which is closely guarded; or they can descend to the bottom of the valley across a half-frozen river and climb a 500-foot-high cliff. They decide to climb the cliff, in the middle of the night, in the middle of winter!

Thanks to Tronstad’s planning and intelligence gathering, they knew every block, stairwell, and entry point. As a result, they were able to infiltrate the plant without an alarm, set the explosives, and blow it up. General Falkenhorst, head of German forces in Norway, called it a “brilliant coup.” [Laughs.]

View Images Joachim Ronneberg, shown here at 93, attends a 2013 wreath-laying ceremony in his honor at the SOE agents monument in London. He is the last surviving member of the Gunnerside team. Photograph by Andrew Cowie, AFP/Getty

Some historians have suggested that the whole campaign against the Vemork facility was not worth the candle. Presumably, you disagree?

I do disagree. For two reasons. First, the Allies didn’t know whether or not Germans were investigating the atomic bomb, but they knew they were intensely interested in heavy water and needed to do everything they could to stop this potential threat.

As it turned out, in the early summer of 1942, Albert Speer decided the Germans were not going to invest in a Manhattan-style nuclear program. But Kurt Diebner, who once headed the program, was told very clearly by his boss, “You get a heavy water reactor going, you prove to us you can do it, and we will shower you with money.”

So Diebner was doing everything he could to get enough heavy water to create an atomic reactor and start the program again. Would they have been able to create a bomb? Probably not. But it is clear that Diebner was very intent on creating a nuclear weapon for the German army.

“War makes the mind very hard,” Leif Tronstad wrote in his diary. “Becoming a sensitive person again will not be easy.” How did the survivors fare after the war?

It was very hard on them, almost to a man. They had lived underground under constant threat for years on end, many of them in isolation. So, when the war ended, it was tough on them. They took it in different ways. Some went to the woods and found solace there; Knut Haugland joined the Kon Tiki expedition, lived on a raft, and found peace there. Others found it in drinking; and others never found it. The sacrifices they made and the hardships they endured affected them for the rest of their lives.

The only survivor of the team today is Joachim Ronneberg. Tell us about your meeting with him—and what he told you about why he wanted to participate.

I never actually met Ronneberg. He was very resistant to talk. But I was fortunate to get the transcript of a 120-page interview with him, done by a Norwegian researcher. What he said was, “You have to fight for your freedom and for peace. You have to fight for it every day, to keep it. It’s like a glass boat; it’s easy to break; it’s easy to lose.”

That is something that I think every one of these individuals felt, and most pointedly, Leif Tronstad. What inspired me about these people was that old cliché about ordinary people doing extraordinary things. These guys included a teacher, a postman, a tour guide, and a factory worker. But when the Germans invade, they decide to do something about it. They happen to be part of this epic mission. But there were hundreds of others, who did equally patriotic work.

This interview was edited for length and clarity.