The letter warned that:
The letter was signed by Einstein on August 2, and delivered to Roosevelt by economist Alexander Sachs. But Sachs was delayed until October 11 because of the president's preoccupation with Hitler's September 1st invasion of Poland, which had started World War II. After hearing Sachs' summary of the letter, Roosevelt authorized the creation of the Advisory Committee on Uranium. The Committee first met on October 21, and was headed by Lyman Briggs, Director of the National Bureau of Standards. $6,000 was budgeted for neutron experiments done by Enrico Fermi at the University of Chicago.
The letter has often been seen as the origins of the Manhattan Project, the successful wartime nuclear weapons project which produced the bombs which were dropped on Hiroshima and Nagasaki in 1945. The path from the letter to the bombings though is considerably longer than just this: the Advisory Committee on Uranium did not vigorously pursue the development of a weapon, and at least two other organizations superseded it (the National Defense Research Committee and the Office of Scientific Research and Development) before the work of fission research was finally superseded by the Manhattan Engineering District in 1942 and became a full-scale bomb development program.
Einstein himself did not work on the bomb project, however, and, according to Linus Pauling, he later regretted having signed this letter.