Though a pacifist, Einstein well understood the Nazi threat; like Szilard and Wigner, he had left Germany because of Adolf Hitler. So the father of relativity signed a letter, prepared primarily by Szilard, to the Belgian ambassador in Washington, warning the Belgian government that bombs of unimaginable power could be made out of uranium, whose primary source was the Belgian Congo. Then Einstein returned to his dinghy, and the two Hungarians drove back to the city.
Szilard worried that this would not be enough: should they not also alert President Franklin Delano Roosevelt? "We did not know our way around in America," Szilard later recalled. But he knew an investment banker named Alexander Sachs, a friend of the president who did know Washington. After Szilard talked to Sachs, the banker concurred: the president must be told.
So two weeks later, on Sunday, July 30, Szilard returned to Einstein's cottage. Wigner was in California, so Szilard -- who did not know how to drive -- turned to another Hungarian, who owned a 1935 Plymouth: a young physics professor at Columbia University named Edward Teller. (Teller would later joke that he entered history as Leo Szilard's chauffeur.) Together Szilard and his bushy-browed driver extracted a second letter from Einstein. It was probably the most important letter of the twentieth century.
"I believe," the greatest scientist of the century wrote to the most important political leader of the age, "it is my duty to bring to your attention . . . that 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 new radium-like elements would be generated. Now it appears almost certain that this could be achieved in the immediate future. This new phenomenon would also lead to the construction of bombs, and it is conceivable -- though much less certain -- that extremely powerful bombs of a new type may thus be constructed. A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory.
"The United States has only very poor ores of uranium in moderate quantities. There is some good ore in Canada and the former Czechoslovakia, while the most important source of uranium is Belgian Congo.
"In view of this situation you may think it desirable to have some permanent contact maintained between the Administration and the group of physicists working on chain reactions in America." (Emphasis added.)
Szilard believed that such a letter, signed by none other than Albert Einstein, would get immediate attention. But it did not. On September 1, 1939, when Hitler attacked Poland, Einstein's letter lay unread somewhere in FDR's in-box.
Einstein's letter was finally brought directly to FDR's attention by Sachs on October 11, and began the process that would lead to the creation of the Manhattan Project -- the top secret government effort to build the atom bomb. But Roosevelt had no idea that the letter was the work of three Hungarian refugees who were not yet American citizens.