But it took years before Mr. Ronneberg came to understand the exact purpose and importance of the job. All the British told him before dropping him onto a snow-covered Norwegian mountain, he said, was that a row of pipes at the Vemork plant needed to be destroyed.

“They just said it was important and had to be blown up,” he said, recalling his wartime exploits in his tidy living room, filled with family photos, including a large framed portrait of his wife, who died last year. The only hint of his past are a few books and magazines in an adjacent study devoted to the history of the war.

He added that he knew nothing at the time about nuclear physics, heavy water or the race to build a nuclear bomb. He knew that Britain had lost more than 35 men in a disastrous 1942 attempt to sabotage the Norsk Hydro plant, but he had no idea why it was so intent on disabling a remote mountain facility whose only product as far as he knew was fertilizer.

“The first time I heard about atom bombs and heavy water was after the Americans dropped the bomb on Hiroshima and Nagasaki” in 1945, he said. “Then we started to understand our raid and why.” And also that, had it failed, London could have ended up “looking like Hiroshima.” This belated realization of the huge stakes at play “was a tremendous satisfaction,” he said.

Historians have long argued over how close Hitler came to developing nuclear weapons. A German historian claimed in a controversial 2005 book that the Nazis conducted several nuclear weapons tests in 1944-45. But a more widely accepted view is that Hitler’s nuclear program, begun well before the Manhattan Project, stumbled badly because of its own inferior science and its enemy’s exceptional saboteurs.

It was handicapped by the flight and murder of Jewish scientists but suffered most gravely from a decision by the physicist Werner Heisenberg to use heavy water, deuterium oxide, instead of graphite, as a so-called moderator in the production of bomb-grade plutonium. Heavy water was not only less effective than graphite but also far more difficult to obtain in sufficiently large quantities, leaving the Nazis dependent on steady supplies from the Norsk Hydro plant in Norway.