Mark Oliphant was a physicist on a mission.

It was 1941 and the Nazis were building, it was believed, a weapon of incredible power. For Britain to beat them to it, American help was essential. But the United States was neutral and its key scientists were being kept in the dark about British progress on the device. So, in late August, Oliphant set out to goad them into action.

Adelaide born and educated, Oliphant had conducted advanced nuclear research at the Cavendish Laboratory in Cambridge under Ernest Rutherford, the man who split the atom. Oliphant was a tireless experimenter who had proved that atomic nuclei could be fused to release energy, and he was a key contributor to Britain’s scientific war effort.

Crossing the Atlantic in an unheated bomber – he considered the Pan-Am Clipper via Lisbon too slow – the dapper, white-haired 39-year-old arrived in the US ostensibly to confer with his American colleagues on radar development. Appalled at the lethargy of the Washington scientific bureaucracy, he wasted no time dropping the “bomb” word. Until then, some of the Americans thought they were working on a power source for submarines.

Exasperated, Oliphant flew to California to speak with his old friend Ernest Lawrence, the US’s youngest Nobel Prize winner and the director of the Radiation Laboratory at Berkeley. Lawrence drove him up “Cyclotron Hill”, site of his still-to-be-built particle accelerator, where, safe from eavesdropping, Oliphant pitched the British view that gaseous diffusion of uranium-235 on a massive scale could lead to the production of a bomb in time to decide the outcome of the war.

Back in Lawrence’s office, they were joined by another Berkeley professor, J Robert Oppenheimer, a leading figure in theoretical physics. Oliphant reiterated the urgent need for American scientists to join forces with the British and get cracking on the bomb project. But this was the first that the intense, contemplative Oppenheimer had heard of the scheme. Already on an FBI watch list due to his left-wing affiliations, Oppenheimer was alarmed to find himself dropped into such a high-level security breach. Clearing his throat, he suggested they end the conversation, since he was not involved with the matter. “But that’s terrible,” Oliphant said. “We need you.”

All this badgering did the trick. Shortly after Oliphant’s return to England, President Franklin D Roosevelt committed the US to the development of a nuclear weapon. Oppenheimer went on to lead the project. By passing secret information to him, Oliphant may have guaranteed Oppenheimer’s involvement. He knew too much to be left out.

Both men came to fear the terrible destructive power they unleashed in 1945. As it turned out, Germany had abandoned the idea of an atom bomb in 1942.