In August of 1939 Einstein mailed a letter to the White House, informing President Franklin Delano Roosevelt of the potential threat posed by the discovery of and subsequent experimentation with nuclear fission in Berlin, Germany. His ominous prediction read:
"This new phenomenon would also lead to the construction of bombs, and it is conceivablethough much less certainthat 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."
History indicates that Einstein sent four letters to President Roosevelt, each expressing an increased urgency for action. In December of 1941, Roosevelt heeded Einstein's warning and convened the American investigation into nuclear fission and the development of such a bomb known as the Manhattan Project. This top secret project went underway in a laboratory in Los Alamos, New Mexico. Four years later, in 1945, the United States dropped the newly-developed atomic bomb, devastating the Japanese cities of Hiroshima and Nagasaki.
Despite his role in alerting the President to the possibility of nuclear weapons, Einstein did not participate in the Manhattan Project. Though he was granted American citizenship in 1940, his involvement with liberal organizations whose missions called for world peace made Einstein a "radical" in the eyes of the Federal Bureau of Investigation. In response to the perceived threat posed by Einstein, the FBI compiled an extensive secret file on the scientist, monitoring and recording his movements. His status as a security threat prevented Einstein from gaining the security clearance necessary to enter the secret laboratory in New Mexico. It is very likely that this was not a source of disappointment for Einstein, who publicly declared his dedication to pacifism. He was quite distressed when the public mind associated him with the dropping of the atomic bombs in 1945 and the subsequent civilian casualties.
A telegram sent to Einstein in 1933, asking for Professor Einstein's caution and discretion when he enters the United States. It requests that he "enter this country quietly and inconspicuously." Courtesy of the Archives of the Institute for Advanced Study, Princeton, New Jersey, USA (268K)