In March 1940 two physicists wrote a top secret memo describing, for the first time, just how to make an atom bomb.
Scientists knew that you could split the nucleus of a uranium atom with a neutron by the end of the 1930s. They also knew that, given the right material in the right amounts, a nuclear chain reaction could be possible. But uranium did not appear to be that material. Sure, you could trigger nuclear fission in uranium, but it also absorbed too many neutrons to allow for a chain reaction.
The answer to this, of course, lay in the difference between uranium 238 and uranium 235. The former is common and does not produce enough fission to spark a chain reaction. The latter is much more rare, but much more reactive. Scientists knew about this too, and also knew how to extract the rare uranium 235 isotope from naturally occurring uranium. But by early 1939 no-one had put all these pieces of knowledge together.
Rudolf Peierls and Otto Frisch were German-Jewish physicists working in Britain. Together, they had a startling realisation. If you could refine uranium to separate out the 235 isotope, you could construct a small device that could sustain a chain reaction. The atom bomb turned from a theoretical possibility to a practical reality.
In early 1940, Peierls and Frisch wrote a top secret memorandum in two parts. One explained the specific mechanisms they had uncovered. The other explained the consequences. The memo is frighteningly prophetic. It anticipates the devastation of a nuclear blast:
This energy is liberated in a small volume, in which it will, for an instant, produce a temperature comparable to that in the interior of the sun. The blast from such an explosion would destroy life in a wide area. The size of this area is difficult to estimate, but it will probably cover the center of a big city.
In addition, some part of the energy set free by the bomb goes to produce radioactive substances, and these will emit very powerful and dangerous radiations. The effects of these radiations is greatest immediately after the explosion, but it decays only gradually and even for days after the explosion any person entering the affected area will be killed.
Some of this radioactivity will be carried along with the wind and will spread the contamination; several miles downwind this may kill people.Frisch-Peierls Memorandum, March 1940
And the policy of nuclear deterrence that would create the Cold War:
As a weapon, the super-bomb would be practically irresistible. There is no material or structure that could be expected to resist the force of the explosion.
If one works on the assumption that Germany is, or will be, in the possession of this weapon, it must be realized that no shelters are available that would be effective and that could be used on a large scale. The most effective reply would be a counter-threat with a similar bomb. Therefore it seems to us important to start production as soon and as rapidly as possible, even if it is not intended to use the bomb as a means of attack.Frisch-Peierls Memorandum, March 1940
One thing they did get wrong, though, was the will of the Allies to use such a weapon:
Owing to the spread of radioactive substances with the wind, the bomb could probably not be used without killing large numbers of civilians, and this may make it unsuitable as a weapon for use by this country.Frisch-Peierls Memorandum, March 1940
The memo triggered the creation of the MAUD Committee, which confirmed the feasibility of this approach. The reports from this committee were greatly influential on the British, American, and (thanks to spies) Soviet nuclear projects. And the rest, as they say, is history.