How Close Were the Nazis to a Nuclear Bomb?

The discovery of nuclear fission in late 1938 and early 1939 immediately raised the prospect of a world filled with dangerous new weapons. Within months, Nazi Germany had banned the export of uranium to other countries and begun looking into the potential military applications of nuclear technology. That effort ramped up following the outbreak of World War II in September 1939. The fear of a Nazi atomic bomb would permeate Allied war plans and prompt much speculation thereafter.

Although it’s unclear how much interest Adolf Hitler took in nuclear matters, several leading Nazis, including Albert Speer, Hermann Göring and Heinrich Himmler, strongly supported uranium research, says Mark Walker, a professor of history at Union College in New York and author of Hitler’s Atomic Bomb: History, Legend, and the Twin Legacies of Auschwitz and Hiroshima. “They hoped there would be a breakthrough.”

Some who had fled Nazi persecution were particularly alarmed. Jewish refugee scientists, in particular, thought that Hitler “was on his way to getting a weapon of mass destruction,” says Tim Koeth, an assistant professor of materials science and engineering at the University of Maryland.

As early as March 1939, Nobel Prize-winning physicist Enrico Fermi—who had recently fled fascist Italy with his Jewish wife—briefed the U.S. Navy on nuclear fission’s potential use as a power source and explosive. Though a small nuclear research grant was subsequently approved, U.S. officials mostly dismissed Fermi’s warning, with at least one officer using an ethnic slur to describe him. “Because he was Italian and had a thick accent, the United States military didn’t take him seriously,” Koeth says.

A few months later, Hungarian Jewish émigré Leo Szilard and German Jewish émigré Albert Einstein wrote to U.S. President Franklin D. Roosevelt, alerting him that Germany might be able to construct “extremely powerful bombs of a new type” and urging him to invest U.S. dollars into nuclear research. Two other Jewish émigrés, German-born Rudolf Peierls and Austrian-born Otto Frisch, sent a similar, albeit more technical, account of a possible “super-bomb” to the British government in early 1940.

Although somewhat late to the game, U.S. officials gradually came to recognize the danger, especially once the United States entered the war in late 1941. “We thought we were in a race to setting the planet on fire, and we had to do it first,” Koeth says. “It was a big concern that Hitler was going to do a surprise attack and obliterate everyone.”

Prior to World War II, scientific information flowed between Allied and German scientists. But once the conflict started, both sides were left in the dark about the other’s nuclear advances, forcing the Americans to “assume the worst,” Walker says. “The Americans were very ignorant of what the Germans were doing, and the Germans were very ignorant of what the Americans were doing,” he says.

Walker says that through the summer of 1942, the American and German programs were essentially comparable. After that, though, the U.S. government began throwing boatloads of resources into the Manhattan Project, just as German nuclear research was grinding to a crawl, due to a combination of shifting priorities and wartime impediments.

Germany’s nuclear ambitions suffered a major setback in February 1943, when Norwegian commandos sabotaged a power plant in Nazi-occupied Norway that served as Germany’s main source of heavy water. (Heavy water can be used as a neutron moderator to sustain a nuclear chain reaction.) Later Allied bombing raids further crippled the Norway plant.

After D-Day, as the U.S. and British armies closed in from the west and the Soviet Army closed in from the east, Hitler was increasingly desperate for “wonder weapons” that would turn the tide of the conflict. To that end, the Nazis explored not just nuclear technology, but everything from jet engines and long-range ballistic missiles (both feasible) to death rays (not feasible). “Obviously, Hitler’s regime was absolutely ruthless, willing to use any weapon,” Walker says.

By late 1944, the clandestine, special-ops Alsos Mission had gathered key intelligence about the German nuclear program. “They rightly come to the conclusion that the German atomic bomb project is nowhere near completion,” Walker says. This prompted Polish Jewish physicist Joseph Rotblat to leave the Manhattan Project on moral grounds. However, most Manhattan Project scientists “didn’t even give it a second thought until they were done,” Walker says. “A few had misgivings.”

Alsos Mission units eventually discovered a German experimental nuclear reactor, which they dismantled, and recovered a drum of nuclear research documents that had been hidden in a cesspool. They also captured and interrogated several German scientists, 10 of whom were then secretly recorded while detained at Farm Hall, an English country house.

Transcripts reveal the German scientists were so stunned upon learning of the atomic bombings of Hiroshima and Nagasaki that they began to berate each other. Up until that moment, they apparently believed they were ahead of their American counterparts.

“All at once, they realized that their work was second class, and that the weapons they were working on for the Nazis were really bad things, and that people were going to be asking them all kinds of questions,” Walker says.

As it turned out, the Germans never even developed a functioning nuclear reactor. A team led by Nobel Prize-winning German physicist Werner Heisenberg conducted one last nuclear experiment in April 1945, just prior to Hitler’s suicide. But the experiment, which involved 664 uranium cubes suspended in heavy water, once again failed to produce the desired results.

By that point, the German nuclear scientists knew the war was lost and that little would come of their program. “The main reason Heisenberg and his colleagues did this experiment was in order to be doing something and avoid being drafted to the front as soldiers,” Walker says.

As Koeth points out, they were also “trying to demonstrate how brilliant they were” to the approaching Allies in hopes of showing off their usefulness and avoiding harsh reprisals. “They were trying to save their own bacon,” Koeth says.

In 1947, Heisenberg claimed that his final wartime reactor experiment was “just insufficient” and lacked only a “relatively small amount of uranium.” But a September 2026 study that Koeth and five colleagues co-authored found him to be wrong. By analyzing two surviving uranium cubes from the German nuclear program, along with historical records and computer modeling, the researchers determined that the Germans would have needed twice as much uranium and about 2.2 times as much heavy water to achieve self-sustaining fission. Those amounts exceeded Germany’s estimated wartime supplies of both materials, the study found.

Some have speculated that Germans should have used graphite as a moderator—as the Americans did—rather than heavy water. But the study concludes that graphite would have failed as well, largely because the coal-based graphite produced in Germany contained far more impurities than the petroleum-based graphite produced in the United States. (Walker argues that the Germans already knew this during the war, which was one reason they turned to heavy water in the first place.)

Even if Heisenberg’s experiment had succeeded, experts point out that achieving a self-sustaining chain reaction was still a long way from building an actual nuclear weapon. In the United States, for example, a team led by Fermi achieved self-sustaining fission in late 1942, but it wasn’t until July 1945—after the war in Europe had already ended—that the first atomic bomb was tested in the New Mexico desert.

“It is unclear if the Germans ever thought of a nuclear bomb [on par with] the Hiroshima and Nagasaki bombs,” says Patrick Park, a graduate student in nuclear engineering at Princeton University and lead author of the 2026 study.

Despite the fears it provoked, Koeth notes that the German nuclear weapons program ultimately proved to be fairly “pitiful.”