As the physicists of the world were discovering that fission could be used to split the uranium atom around 1939 scientists in Britain, France, Germany, Japan, the USSR and the United States all began to work on the possibility of making an atomic bomb. For a long time there was great skepticism among US scientists that the bomb could be made to be with a reasonable amount of atomic material but British scientists made the breakthrough calculations that convinced the US team that it could be feasible. Many scientists the in US were very resistant to make their atomic discoveries secret because the world of science had always been one of complete openness to all scientists. However as the possibility of a bomb was becoming more real the US scientists finally made all their work secret. The USSR was convinced (correctly) the the United States was working on a bomb when some of their scientists announced a very important observation and there was not a single response from a United States scientist.

Book – The making of the Atomic Bomb – Richard Rhodes)