After the discovery of Fermin, Szilard joined him in New York. Together they figured that a kilogram of uranium would make energy similar to 20,000 tonnes of TNT. However, Szilard knew that this discovery could be disastrous and feared that the Nazis would soon figure this out too.

In 1940, two German physicists working in the UK worked out how to produce uranium-235 in high quantities, how it could be used to make a bomb, and the disastrous consequences if they succeeded.

In March 1940, German physicists Rudolf Peierls and Otto Frisch wrote to the British government in their Memorandum on the Properties of a Radioactive 'Super-Bomb', urging them to take action. It led to the British bomb project, codenamed Tube Alloys.

The US government appointed physicist Arthur Compton to lead a nuclear weapons programme, eventually named the Manhattan Project. Over the next four years, the US, UK and Canada poured resources into the Manhattan Project. Tube Alloys was eventually joined with the US project.

In 1942, a pile of uranium and graphite was assembled in a squash court underneath a stand on a football field in Chicago. Then, on 2 December 1942, the first man-made nuclear chain reaction was created. Once confirmed, Szilard shook hands with Fermi and said, “This will go down as a black day in the history of mankind.”

On 16 July 1945, the US detonated the world’s first nuclear bomb in the New Mexico desert. The test proved that nuclear energy could be weaponised.