Overview: The concept of splitting the atom and its possible implications was known to scientists across the world before the outbreak of the Second World War. President Franklin Roosevelt had been alerted to the energy generated by a nuclear chain reaction as early as August 1939, in a letter signed by Albert Einstein, which warned of the destructive possibilities of a bomb which harnessed such technology. Although Einstein did not participate in the project, not least due to his pacifist inclinations, he was terrified of the possibility of Nazi Germany developing such a weapon. Roosevelt took the warnings seriously, setting up an advisory committee to report further, but it was not until the United States’s entry into the conflict in December 1941 that decisive action was taken.This episode tells the story of the first testing of the nuclear fission bomb, code-named Trinity, in the New Mexico desert.