When the three leaders of the victorious allies, Britain, the United States and the Soviet Union, met at Potsdam in July 1945, President Truman announced to Stalin that the US had a new and devastating weapon. Observers report that Stalin paid no attention to this remark. In fact, Stalin was well aware of the existence of the atomic bomb, and the Soviet Union was rapidly developing its own. Stalin owed his knowledge to the atomic scientist Dr Klaus Fuchs, who can lay claim to being the most successful spy in history. A refugee from Nazi Germany, entrusted with crucial work at the very heart of the British and American nuclear weapons project, Fuchs gave every piece of information he had to the KGB, the Russian intelligence agency. Then in 1950, his spy mission complete, he made an unprompted confession to MI6. His espionage accelerated the start of the Cold War between Russia and the West, and caused a split between the US and British governments that forced Britain to build its own atomic weapons. The world that Fuchs helped create remained in the grip of a nuclear stand off for a generation. The Spy Who Changed the World uses previously unseen archive documents to bring to life one of the most compelling spy stories on the twentieth century.