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Einstein's 1939 letter helped initiate the U.S. effort to build an atomic bomb, but work proceeded slowly at first. Two other findings in 1940 and 1941 demonstrated conclusively that the bomb was feasible and made building the bomb a top priority for the United States: the determination of the "critical mass" of uranium needed and the confirmation that plutonium could undergo fission and be used in a bomb. In December 1941, the government launched the Manhattan Project, the scientific and military undertaking to develop the bomb.

Einstein wrote to U.S. President Franklin Roosevelt to warn him that the Nazis were working on a new and powerful weapon: an atomic bomb. Fellow physicist Leo Szilard urged Einstein to send the letter and helped him draft it.

July 1940, the U.S. Army Intelligence office denied Einstein the security clearance needed to work on the Manhattan Project. The hundreds of scientists on the project were forbidden from consulting with Einstein, because the left-leaning political activist was deemed a potential security risk.