How Nazi secrets triggered the ultimate moral dilemma – and led to Oppenheimer’s atomic bomb

But as physicist Frank Close explains on the HistoryExtra podcast, this story began long before wartime Los Alamos. It began in Britain, and it began with the question: what if the Nazis got there first?

“There is no defence… other than to have one yourself”

In early 1939, news of a startling discovery had begun to ripple through the international physics community.

In Germany, scientists Otto Hahn and Fritz Strassmann had shown that bombarding uranium atoms with neutrons could cause them to split – releasing huge amounts of energy.

The implications were obvious to physicists. When a neutron hits a uranium nucleus, the atom splits and can emit more neutrons. If those neutrons strike other uranium atoms, a chain reaction begins – each fission releasing heat and radiation.

Harnessed and controlled, this new research created a theoretical pathway to developing a weapon capable of unimaginable destruction. But could it actually work?

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In Britain, two physicists – Otto Frisch and Rudolf Peierls, both refugees fleeing from the clutches of Nazism – decided to investigate.

“Frisch and Peierls did the calculation,” Close explains, “and discovered, to their shock and horror, that you only needed to make about the size of a grapefruit of uranium-235, and that would have an explosive power equivalent to a thousand tonnes of dynamite.”

A weapon that size could be delivered by a single bomber, yet do the same damage as the Allied firebombing of Dresden inflicted during the end of the war, several years later – a campaign that involved hundreds of aircraft over multiple nights.

“There is no defence against such a weapon other than to have one yourself,” says Close. “And they think: what if the Nazi scientists have already realised this themselves? Could Germany be building a bomb right now?”

It was that terrifying prospect that triggered the race to build the bomb.

Oppenheimer: father of the bomb, or midwife?

Thanks in part to Christopher Nolan’s 2023 film Oppenheimer, J Robert Oppenheimer is now the most recognisable face behind the development of nuclear weapons. But Close argues that this distorts the timeline – and overlooks Britain’s foundational role.

In fact, by the time the United States entered the war in December 1941, British scientists had already done much of the key theoretical work. “Oppenheimer is often described as the father of the atomic bomb,” Close adds. “[But] I think he was the midwife of the atomic bomb – not the father.”

Tube Alloys: Britain’s top-secret bomb project

In 1940, with Nazi bombers looming over British cities and the threat of invasion ever-present, Frisch and Peierls’ findings prompted immediate action. The British government launched a secret atomic programme, under the bland-sounding codename Tube Alloys.

“This was a secret that was taken action on solely in the United Kingdom,” Close says. “And it began a project pursuing ways of enriching uranium.”

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That meant isolating uranium-235 – the rare isotope that could sustain a chain reaction – from uranium-238, which was more abundant but didn’t support fission. Natural uranium contains just 0.7 per cent uranium-235.

British scientists focused on a method called gaseous diffusion, in which uranium gas is passed through a series of membranes to gradually increase the concentration of U-235. It was technically demanding – but in theory, possible.

At this point, the United States, still neutral, saw nuclear physics as a field for peaceful research – a means of developing new energy systems rather than weaponry.

British science meets American industry

That soon changed. As Nazi victories mounted and Britain stood alone in Europe, Winston Churchill sought deeper scientific and military cooperation with the United States. In September 1940, a delegation of British scientists crossed the Atlantic armed with technical briefings – including key data on uranium enrichment.

The collaboration took on a new dimension, however, following the Japanese attack on Pearl Harbor on 7 December 1941. The US entered the war the following day, and British theory combined with America’s vast industrial capacity. By 1943, British physicists including Rudolf Peierls had relocated to the US to help build enrichment plants at Oak Ridge, Tennessee.

“That became the huge laboratory where the uranium-235 was created, which eventually became the fuel of the bomb that was dropped over Hiroshima,” Close explains.

Trinity and the two bombs

While work on the uranium bomb advanced, scientists also pursued a second design using plutonium – a man-made element created in nuclear reactors. Unlike uranium-235, plutonium required a more complex implosion mechanism to trigger detonation.

To prove it would work, they built and tested a prototype. The result was the Trinity test, conducted in New Mexico on 16 July 1945.

“The Trinity test was the moment they tested the plutonium weapon,” says Close. “And it indeed exploded. That was when the atomic age really began.”

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A few weeks later, two bombs were dropped: Little Boy, a uranium bomb, on Hiroshima; and Fat Man, a plutonium bomb, on Nagasaki. Japan surrendered soon after. The Second World War was over, and the nuclear age had begun in earnest.

As for the scientists working on the project, the stark reality of what they had created became immediately obvious. “They realised they had discovered a completely new weapon of warfare.”

A moral gamble in the fog of war

In the end, Germany and the other Axis forces never built a bomb. While some Nazi physicists investigated fission, they never made the same conceptual leap as Frisch and Peierls. Nor did researchers in Japan or Italy.

The only other scientists who came to the same insights were in the Soviet Union – and their research was kept secret for decades.

But none of that was known in 1940. And, as Close argues, that uncertainty – the ambiguity as to whether the enemies of the Allies also possessed this terrible power – meant they were faced with an unenviable dilemma: what if we don’t act, but they do?

Frank Close was speaking to Matt Elton on the HistoryExtra podcast. Listen to the full conversation.