The building by the United States of an atomic bomb was not the result of a single decision, but of a series of decisions taken over more than two years. Although President Franklin D. Roosevelt held the ultimate responsibility, his attitudes were shaped by scientific advisers whose reasoned conclusions and best guesses persuaded him that it was possible to construct a nuclear fission device “of superlatively destructive powers,” as a 1941 report termed it.
The building by the United States of an atomic bomb was not the result of a single decision, but of a series of decisions taken over more than two years. Although President Franklin D. Roosevelt held the ultimate responsibility, his attitudes were shaped by scientific advisers whose reasoned conclusions and best guesses persuaded him that it was possible to construct a nuclear fission device “of superlatively destructive powers,” as a 1941 report termed it.
Research had begun during the 1920’s and 1930’s, primarily by European physicists, including James Chadwick in Great Britain, Enrico Fermi and Emilio Segrè in Italy, Lise Meitner and Otto Frisch, who in 1938 fled Austria for Denmark, (where Niels Bohr was working), Hungarians such as Leo Szilard, the Frenchman Frédéric Joliot-Curie, and Otto Hahn and Fritz Strassmann at the Kaiser Wilhelm Institute in Berlin. Their research indicated the possibility of bombarding the nucleus of the uranium atom, splitting it into lighter fragments, and releasing tremendous amounts of energy. A significant number of these scientists fled fascism for the United States or England. Many of them gathered with U.S. physicists in January, 1939, at the fifth Washington Conference on Theoretical Physics to hear Bohr recount the exciting atomic discoveries. Within the year, nearly one hundred papers had been published in scholarly journals expanding on and confirming this new work.
In March, 1939, Fermi, Szilard, and a number of other émigré physicists who feared that the Germans were developing an atomic bomb began a lengthy effort to arouse in both their U.S. colleagues and the United States government some sense of their own urgent concern. After Fermi’s direct approach to the U.S. Navy on March 17 failed to generate any active interest, and after the Germans forbade further export of uranium ore from the Joachimstal mines in recently conquered Czechoslovakia, Szilard became convinced that Albert Einstein was the only scientist in the United States with enough fame and prestige to garner a sympathetic hearing from the U.S. government. During a visit to Einstein on Long Island in mid-July, 1940, Szilard exacted from his old friend a promise to write, or at least sign, any letter or letters that might be needed to attract the attention of the U.S. government. With Einstein’s promise in hand, Szilard and fellow émigré physicist Eugene Wigner wrote a letter addressed to President Roosevelt. Dated August 2, 1939, and signed “A. Einstein,” this letter, detailing the dangers and possibilities of atomic energy, was presented to Roosevelt on October 11 by Alexander Sachs, an occasional presidential adviser who had eagerly agreed to serve as the intermediary for Szilard.
Sachs and the Einstein letter convinced the president that the situation should be explored. Accordingly, he established the Advisory Committee on Uranium. Headed by Lyman Briggs, director of the National Bureau of Standards, and including representatives from the Army, the Navy, and the scientific community, this attempt to draw federal support into scientific research for the national defense produced few early results. The committee met infrequently, and its financial support involved only a six-thousand-dollar research grant.
Research on the explosive potential of uranium, which was being conducted at university laboratories scattered across the country, pointed in two main directions. One involved the separation of the fissionable isotope U-235 from the much more common U-238 by a variety of methods, including gaseous or thermal diffusion, electromagnetic separation, and the centrifuge. The other sought to transmute uranium into a new fissionable element, plutonium (U-239), through a controlled chain reaction in an atomic pile. It was not until 1942 that either a chain reaction or the separation of more than a few micrograms of U-235 would be accomplished.
As the Germans drove into France in May and June, 1940, others in the scientific community, including Hungarian-born émigré physicist Edward Teller, grew increasingly concerned. Responding to that concern, on June 15, President Roosevelt established the National Defense Research Council (NDRC) under the leadership of Vannevar Bush, president of the Carnegie Institute. Creative and highly capable, Bush and his able deputy, Harvard president James Conant, played key roles in the decision to make the bomb.
While support for the Advisory Committee on Uranium and other scientific defense research grew during the next year, Bush believed that the work lacked the necessary urgency. On June 28, 1941, acting on Bush’s advice, Roosevelt created the stronger Office of Scientific Research and Development (OSRD), with Bush as the head. Conant moved up to head the NDRC, and the Uranium Committee, strengthened and enlarged, became the S-1 Section of OSRD.
Although the establishment of OSRD represented a significant organizational step, it did not signify a decisive commitment to the building of an atomic bomb. Key figures in the U.S. government–Roosevelt, Vice President Henry Wallace, Secretary of War Henry L. Stimson, and Army Chief of Staff George C. Marshall–members of the OSRD, and members of the U.S. scientific community remained skeptical about both the cost and feasibility of developing an atomic weapon. This skepticism, however, began to give way during the second half of 1941. At that time, the British government, based on the recent ideas of Otto Frisch and Rudolf Peierls, refugee physicists working at Cambridge, reported to the OSRD its belief that an atomic bomb could be developed within two years. Another push to the U.S. atomic effort was provided by Mark Oliphant, the Australian-born head of the physics department at the University of Birmingham. During a visit to the United States in August, 1941, Oliphant pressed upon Bush the British conviction that a bomb really could be made.
Oak Ridge National Laboratory in Tennessee in 1944, a year after it was built. The lab’s contribution to the Manhattan Project was production of the plutonium needed to produce atomic bombs. (Martin Marietta)
With the Japanese attack on Pearl Harbor and the German and Italian declarations of war on the United States in December, 1941, Roosevelt had to choose between committing to the construction of a weapon that might win the war in the long run or cutting back on an unproven program to concentrate valuable resources to the more immediate goal of not losing the war in the short run. On March 9, 1942, Bush informed the president that a major industrial effort might produce an atomic weapon in 1944, but that a decision had to be made soon. After receiving additional encouraging news, Roosevelt decided on June 17, 1942, that the United States would build an atomic bomb.
Having committed itself to the construction of an atomic weapon, the U.S. government had to determine how to produce sufficient quantities of fissionable materials. After learning from S-1 Section researchers that four methods–gaseous diffusion, the centrifuge, electromagnetic separation, and controlled chain reactions in uranium piles–were at comparable stages of development, it was decided to make an all-out effort on all four fronts, rather than explore a single method that might prove a dead end.
The U.S. atomic bomb program–code-named the Manhattan Project and headed by General Leslie R. Groves (appointed September 17, 1942)– involved highly secret research at Los Alamos, New Mexico, where basic bomb development took place; Oak Ridge, Tennessee, where U-235 was separated from U-238 by gaseous diffusion and electromagnetic techniques; and Hanford, Washington, where plutonium was produced in graphite piles. At a cost of nearly two billion dollars, the Manhattan Project ultimately paid dividends: The first bomb was successfully detonated on July 16 at Alamogordo, New Mexico, and there followed production of the weapons that ended World War II in August, 1945, and that enabled the United States to lead the world into the Atomic Age.