No single decision created the atomic bomb, but most accounts begin with the presidential discussion of a letter written by Albert Einstein. ‘Building the bomb’ describes how the bomb came about, from Einstein's letter to Roosevelt about the threat of nuclear weapons to the bombings in Japan. What were the ramifications of the atomic bombs? The impact of the Manhattan Project’s new weapon spread well beyond military and scientific circles, to an extent unprecedented in the popular imagination. A turning point in the history of the contemporary world had been reached. ‘The bomb’, as it was dubbed, became a defining feature of the post-World War II world.