Klaus Fuchs knew more nuclear secrets in the last two years of the Second World War than anyone else in Britain. He was taken onto the Manhattan Project in the USA as a trusted physicist - and was the conduit by which knowledge of the highest classification passed to the Soviet Union. When Truman announced at the Potsdam Conference that the US possessed a nuclear bomb, Stalin already knew. This book, by an accomplished scientist as well as historian, explains the physics as well as the spying, and because Frank Close worked, like Fuchs, at the Harwell Laboratory, it contains much important new material.