They were going to sabotage the Nazi atomic bomb. But only Capt. Leif Tronstad, Sr. knew it.

A week before Christmas, 1942, Tronstad gathered the six men he was about to send behind German lines as they prepared to ship out. Like him, the commandos of Operation Gunnerside were all Norwegians, exiled to the U.K. after the Nazis conquered their country two years before. Their target was a key part of the German nuclear program, but it was nowhere near the Kaiser Wilhelm Institute outside of Berlin, where a Nobel prize-winning physicist was trying to build a uranium reactor. It wasn't anywhere else in Germany, for that matter. They were going home.

The Norwegians would leave the comfort of pastoral Brickendonbury Hall, located north of London, for the ice- and wind-ridden mountains of their native land. The long night of winter would cover them as they parachuted onto a plateau above a small factory town called Rjukan, which produced an ultra-rare kind of water that was a byproduct of fertilizer production. Then they would ski down to the plant and destroy it.

, a new book about the string of Allied raids on the Nazi heavy water supply, recounts what Tronstad said next:

It was a solemn moment. Each of the saboteurs had been given a cyanide capsule; each knew that their chances of hitting the target and escaping with their lives were, at best even."You do not know now exactly why it's so important, but trust that your actions will live in history for a hundred years to come. What you do, you do for the Allies," he said. "And for Norway."

Sudden Interest

Norwegian King Haakon VII and Tronstad, in exile in Britain. The Winter Fortress/Norges Hjemmefrontmuseum

Heavy water is H20 but with an uncharacteristic neutron added to both hydrogen atoms. This stuff is everywhere in nature, mixed in with normal water at a concentration of 1 part per 41 million. That paltry concentration can be increased by running an electrical current through water, because the heavy stuff doesn't split into hydrogen and oxygen gases as easily and normal water molecules.

Rjukan, Norway, was home to the world's finest heavy water reactor, a cascading tower of electrolysis chambers where heavy water molecules would fall, Plinko-like, downward until they reached a vessel that held 99.5 percent pure heavy water. Making each gallon of heavy water required tens of thousands of gallons of water and thousands of kilowatt hours of electricity, enough to power a small town for a year. Norsk Hydro, the company that owned the Vemork factory in Rjukan, started making the stuff since the early '30s, but it had been nothing more than a scientific curiosity until German scientists discovered nuclear fission in 1938. Over the next several years, the Third Reich developed an unquenchable thirst for it. The Norwegians were the only supplier around.

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Before the war, Tronstad had been a star chemist with deep firsthand knowledge of atomic physics gained from stints at Cambridge University where he studied under Ernest Rutherford, a godfather of nuclear physics. Tronstad realized why the Germans were so interested in heavy water. Scientists recognized the stuff could be a useful moderator—that is, a material which would neither dampen a chain reaction before could go critical, nor let it fizzle out prematurely. To this day it remains a solid choice for uranium reactors, which are the first step on the path to a bomb. For the same reason the US and its allies want to deny Iran access to heavy water today, Tronstad wanted to destroy Vemork.

Tronstad had more than just technical expertise. His rich informant network had alerted the Allies to Germany's intense interest in heavy water. "America put everything into the Manhattan Project partly — not wholly, but partly — because of information Leif Tronstad was giving the Allies on the intensity of the German atomic project," says Neal Bascomb, the author of The Winter Fortress. All of this made him an indespensible asset—and too valuable to make the jump into Norway alongside his commandos, though he desperately wanted to.

Then they would ski down to the plant and destroy it.

Consigned to devising a plan, Tronstad set his obsessive, detail-oriented mind to work. Entry points, hallway lengths, staircase heights, objectives, guard patrol schedules—all were provided in excruciating detail. Every contingency was accounted for.

After all, who better to destroy the Vemork heavy water reactor than the man who'd built it?

Operation Gunnerside

Aerial view of Vemork, pipes extending up behind it. The Winter Fortress/Norges Hjemmefrontmuseum

When the Germans invaded Norway in April 1940, Leif Tronstad committed everything to taking back his country. "[F]amily, house and worldly goods had to be set aside for Norway's sake," he wrote in his diary on September 22, 1940, the day he went on the run, Gestapo hot on his trail. That list would grow to include the destruction of his masterpiece.

Tronstad's vision and execution built Vemork's heavy water reactor. Snowmelt tumbling down from the mountains provided both the water and the energy required to separate heavy water. In 1933 it was the cutting edge of science; heavy water had only been isolated in any amount a year before. "He just wanted to see if he could build it," Bascomb says. Working with a Norsk Hydro employee named Jomar Brun, Tronstad had designed new electrolysis chambers to capture as much heavy water as possible, distilling it to an almost pure concentration.

The Germans knew Tronstad's background. They knew he was in the U.K. working with the Allies. They knew the Vemork heavy water plant had a big red target on it, all of which made the mission's success that much more remarkable.

After a post-jump rendezvous with a Norwegian resistance cell, Tronstad's team, now nine men strong, snuck onto the plant by climbing down into steep ravine and back up its icy cliffs. Tronstad knew they didn't have to destroy the entire factory, just the heavy water reactor in the basement. Making the most of technical plans and intel smuggled out of Norway, his plan called for the four-man demolition squad to creep in through a shaft for pipes and cables that led directly to their target. It was their best chance of going undetected, and besides, everything else was locked shut. The rest of the team posted up outside the guard house manned by German soldiers. They crouched in the shadows, tommy guns and hand grenades at the ready.

"He just wanted to see if he could build it."

But the only resistance, if you can call it that, was a Norwegian plant worker sitting at a desk in the basement. While one commando made sure he kept quiet, the others rigged the heavy water tanks with explosives and a 30-second fuse. The covering squad was so underwhelmed by the explosion, they questioned whether it had succeeded. It had. By the time the Germans figured it out, the Norwegians had escaped back into the wilderness. None of Tronstad's commandos needed their cyanide pills. All of them made it out of the mission alive.

Operation Gunnerside did not demolish all of Hitler's atomic aspiration. The Germans quickly repaired the facility and, as detailed in Bascomb's book, the Allies launched several more raids to disrupt the heavy water supply. (Nobel laureate and German atomic project member Werner Heisenberg lamented the Norwegians had made a national sport of such sabotage.)

These attacks were not the sole reason the U.S. succeeded in developing an A-bomb by war's end and the Germans did not. "The lack of heavy water was a big problem stopping everybody, but the German bomb project had enough other problems that this was not the single bottleneck," says Richard Kremer, a history professor at Dartmouth College and an expert on science in Nazi Germany. Uranium and even basic laboratory supplies were also difficult to procure, he says. Like the rest of the Nazi regime, Germany's bomb project often looked more like warring factions jockeying for position rather than a centralized, concerted effort like the Americans had.

Maybe Germany's bomb research was doomed anyway. Nevertheless, the German high command had promised to shower resources upon Heisenberg and his colleague/rival Kurt Diebner if they could get a uranium reactor working. They never did, thanks in part to Leif Tronstad. And while the heavy water saga may have been his greatest contribution to the Allied cause, it wasn't his greatest contribution to Norway's.

Saving Norway

Tronstad (center) with the team after Gunnerside mission. The Winter Fortress/Norges Hjemmefrontmuseum

Tronstad thought his final act in the war would be the most important. That's according to his son, Leif Tronstad, Jr., who is still living. When the tide of World War II turned against Germany, Tronstad's goals shifted. Instead of sabotaging his own country's technology to limit its contributions to Germany's war capacity, he set about doing whatever he could to prevent the Nazis from trashing the place on their way out.

Tronstad finally got his wish to take the fight to the Nazis when the Allies approved "Operation Sunshine," his plan to secure critical infrastructure before the Germans could enact scorched earth tactics. In late 1944, Tronstad parachuted into the same mountains his Gunnerside commandos had two years earlier. He had made it back to his beloved Norway, but he would never see his family again.

On March 11, 1945, Tronstad was interrogating a Nazi sympathizer who was sheriff of the area around Vemork. He and his men had lured the sheriff out to a cabin and ambushed him. After questioning him, they debated killing the sheriff before deciding to simply take him prisoner.

Uncharacteristically, Tronstad made two fateful mistakes.

First, he failed to account for a critical variable: the sheriff's brother. Following their tracks, the brother found the cabin, where Tronstad had failed to leave a sentry to keep watch. Second mistake. The brother burst into the cabin and began firing a rifle. A melee broke out and in the commotion, Tronstad was killed, either by a bullet or rifle butt to the head, depending on the account. The sheriff and the brother survived and hid the body in a nearby lake. Later, they were tried and convicted for Tronstad's death.

Finally, Fame

Photos from Leif Tronstad Sr.'s time in the UK, a military training camp in Scotland in the summer of 1944 prior to returning to Norway for Operation Sunshine. Leif Tronstad Jr.

While Tronstad earned some posthumous fame immediately after the war, his name was sidelined for sixty years, even in accounts of Operation Gunnerside. But in 2012, Tronstad got the prestige treatment in " ," a Norwegian-produced miniseries in the vein of "Band of Brothers" that made him a main character. For his book, Bascomb uncovered even more about the man thanks to finding never-before-published correspondence and excerpts from the diary. With a feature film based on The Winter Fortress already in the works, Leif Tronstad's name may soon be uttered in movie theaters around the world. "He was pretty much a pure, unalloyed hero," Bascomb says.

A full accounting of the decisions war forces people to make often shreds any attempts at hagiography. But at a time when Nobel prize-winning scientists on both sides reckoned with their contributions to the most ghastly weapon ever conceived, Tronstad stands out. Operation Gunnerside was a meticulous mission plan aimed to prevent collateral damage in his homeland, especially compared to what an air raid would (and later did) bring.

"He was pretty much a pure, unalloyed hero."

And then there was his family. During the Nazi occupation of Norway, some families were extracted to the U.K. or neutral Sweden. But while Tronstad might have been able to protect his own family that way, he had dozens of men under his command for whom that was impossible. Tronstad was the sort of commander who wouldn't allow for himself what he had to deny others. So his wife, Bassa, stayed behind with their two young children, and was interrogated numerous times by the Gestapo. Leif Tronstad, Jr. was young at the time but has come to understand his father's thought process. "She couldn't have very much to tell [the Gestapo] because she didn't know a lot," he said.

"He was always making human considerations," Tronstad, Jr. says. "The man behind the decision is always shining through. That shines through all the time."

"The Winter Fortress" is available in hardcover from Houghton Mifflin Harcourt. "The Heavy Water War" is available on DVD from the Norwegian Broadcasting Company and on Netflix.

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