Oppenheimer and many other physicists around the country had known as early as February 1939 that an atomic bomb was a real possibility. But arousing the government’s interest in the matter would take time. A month before war broke out in Europe (September 1, 1939), Leo Szilard had persuaded Albert Einstein to sign his name to a letter (written by Szilard) addressed to President Franklin Roosevelt. The letter warned the president “that extremely powerful bombs of a new type may be constructed.” He pointed out that “a single bomb of this type, carried by boat and exploded in port, might very
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