The development of a nuclear weapon was a low priority for the United States before the outbreak of World War II. However, scientists exiled from Germany had expressed concern that the Germans were developing a nuclear weapon. Confirming these fears, in 1939 the Germans stopped all sales of uranium ore from the mines of occupied Czechoslovakia. In a letter sponsored by group of concerned scientists, Albert Einstein informed President Roosevelt that German experiments had shown that an induced nuclear chain reaction was possible and could be used to construct extremely powerful bombs (7; 12)*. * All sources cited in the text are listed alphabetically in the reference list at the end of this volume. The number given in the text corresponds to the number of the source document in the reference list. In response to the potential threat of a German nuclear weapon, the United States sought a source of uranium to use in determining the feasibility of a nuclear chain reaction. After Germany occupied Belgium in May 1940, the Belgians turned over uranium ore from their holdings in the Belgian Congo to the United States. Then, in March 1941, the element plutonium was isolated, and the plutonium-239 isotope was found to fission as readily as the scarce uranium isotope, uranium-235. The plutonium, produced in a uranium-fueled nuclear reactor, provided the United States with an additional source of material for nuclear weapons (7; 12). In the summer of 1941, the British Government published a report written by the Committee for Military Application of Uranium Detonation (MAUD). This report stated that a nuclear weapon was possible and concluded that its construction should begin immediately. The MAUD report, and to a lesser degree the discovery of plutonium, encouraged American leaders to think more seriously about developing a nuclear weapon. On 6 December 1941, President Roosevelt appointed the S-1 Committee to determine if the United States could construct a nuclear weapon. Six months later, the S-1 Committee gave the President its report, recommending a fast-paced program that would cost up to $100 million and that might produce the weapon by July 1944 (12). The President accepted the S-1 Committee's recommendations. The effort to construct the weapon was turned over to the War Department, which assigned the task to the Army Corps of Engineers. In September 1942, the Corps of Engineers established the Manhattan Engineer District to oversee the development of a nuclear weapon. This effort was code-named the "Manhattan Project" (12).