In 1943 fear that the German war machine might use atomic bombs was abating and among physicists another fear was taking its place - that of a postwar nuclear arms race with worldwide proliferation of nuclear weapons. Manhattan Project scientists and engineers began to discuss uses of nuclear energy in the postwar world. Niels Bohr, Leo Szilard, James A. Franck and others launched a concerted effort to lay groundwork for international control of the technology. Realizing the devastation nuclear weapons could cause and that they could be made and delivered much more cheaply than conventional weapons of the same power, they tried to persuade policy makers to take into account long range consequences of using atomic bombs and not base their decisions on short range military expediency alone. They met with little success. The scientists' main message, unheeded then and very relevant now, is that worldwide international agreements are needed to provide for inspection and control of nuclear weapons technology. Their memoranda and reports remain as historic documents eloquently testifying to their concern.