"Some recent work by E. Fermi and L. Szilard... leads me to expect that the element uranium may be turned to a new and important source of energy in the immediate future. Certain aspects of the situation call for watchfulness and, if necessary, quick action on the part of the administration..... In the course of the last four months it has been made... possible to set up a nuclear chain reaction in a large mass of uranium, by which vast amounts of power... 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. However, such bombs might very well prove too heavy for transportation by air... 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."