Enrico Fermi, member of the scientific team working on the Manhattan Project, achieved the world's first sustained and controlled nuclear chain reaction at the University of Chicago on December 2, 1942, which led to the creation of the first atomic bomb. Source-Wikipedia
Our government. On August 2, 1939 just before the beginning of World War 2 Albert Einstein and several other scientist warned Franklin D Roosevelt of Nazi Germans attempt to purify uranium-235. This could be used to create an atomic bomb. It was shortly after this that the United States Government began the serous plans know only as "The Manhattan Project".