Today in 1939, a letter written by Leó Szilárd and signed by Albert Einstein was sent to President Franklin D. Roosevelt, informing him of the possibility of constructing an atomic bomb and that Germany is conducting research on splitting uranium while stopping its exports from Czechoslovakia:
…it may become possible to set up a nuclear chain reaction in a large mass of uranium, by which vast amounts of power and large quantities of radium-like elements may be generated…. This new phenomenon would also lead to the construction of bombs, and it conceivable—though much less certain—that extremely powerful bombs of a new type may thus be constructed.
The letter started the process of bomb development that eventually was called the Manhattan Project.