United States Develops the First Nuclear Weapon Summary

  • Last updated on November 10, 2022

The United States designed and built the world’s first nuclear weapon in a highly secretive U.S. government research program called the Manhattan Project.

Summary of Event

The building by the United States of an atomic bomb came after a series of decisions made during a period of 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. Manhattan Project Nuclear weapons;invention Atomic bomb [kw]United States Develops the First Nuclear Weapon (June 17, 1942-July 16, 1945) [kw]Nuclear Weapon, United States Develops the First (June 17, 1942-July 16, 1945) [kw]Weapon, United States Develops the First Nuclear (June 17, 1942-July 16, 1945) Manhattan Project Nuclear weapons;invention Atomic bomb [g]North America;June 17, 1942-July 16, 1945: United States Develops the First Nuclear Weapon[00540] [g]United States;June 17, 1942-July 16, 1945: United States Develops the First Nuclear Weapon[00540] [c]Inventions;June 17, 1942-July 16, 1945: United States Develops the First Nuclear Weapon[00540] [c]Physics;June 17, 1942-July 16, 1945: United States Develops the First Nuclear Weapon[00540] [c]Science and technology;June 17, 1942-July 16, 1945: United States Develops the First Nuclear Weapon[00540] [c]World War II;June 17, 1942-July 16, 1945: United States Develops the First Nuclear Weapon[00540] [c]Military history;June 17, 1942-July 16, 1945: United States Develops the First Nuclear Weapon[00540] Bush, Vannevar Conant, James Bryant Roosevelt, Franklin D. [p]Roosevelt, Franklin D.;Manhattan Project Sachs, Alexander Szilard, Leo

Research had been going on in the 1920’s and 1930’s, primarily by European physicists, including James Chadwick in Great Britain; Enrico Fermi and Emilio Gino 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. Nuclear physics 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 Nazis were developing an atomic bomb began a lengthy effort to arouse in both their U.S. colleagues and the U.S. government some sense of their own urgent concern. After a direct approach by Fermi to the U.S. Navy, made 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 Einstein, Albert was the only scientist in the United States with enough fame and prestige to garner a sympathetic hearing from the U.S. government.

Visiting 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. 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 Advisory Committee on Uranium . Headed by Lyman Briggs Briggs, Lyman , director of the National Bureau of Standards, and including representatives from the U.S. 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 $6,000 research grant.

Research on the explosive potential of uranium, which was being conducted at some twenty university laboratories scattered across the country, pointed in two main directions. One involved the separation of the fissionable isotope Radioisotopes Nuclear fission 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 National Defense Research Council, U.S. (NDRC) under the leadership of Vannevar Bush, president of the Carnegie Institute. Creative and highly capable, Bush and his able deputy, Harvard president James Bryant 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 Office of Scientific Research and Development, U.S. (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 the 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 Oliphant, Mark , the Australian-born head of the physics department at the University of Birmingham. During a visit to the United States in August, Oliphant pressed upon Bush the British conviction that a bomb really could be made.

With the Japanese attack on Pearl Harbor and the German and Italian declarations of war on the United States in December, 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 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 Richard Groves Groves, Leslie Richard (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 $2 billion, 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, not without controversy, ended World War II in August, 1945, and that enabled the United States to lead the world into the troubled atomic age.

The atomic age held the promise of exploring this new source of energy for peaceful purposes, but it also produced the threat of global nuclear annihilation during the Cold War with its attendant nuclear arms race. Given this double-edged quality of nuclear technology, some participants in the development of the nuclear bomb later founded or participated in organizations devoted to diminishing the threat of nuclear war. Manhattan Project Nuclear weapons;invention Atomic bomb

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Gaddis, John Lewis, et al., eds. Cold War Statesmen Confront the Bomb: Nuclear Diplomacy Since 1945. New York: Oxford University Press, 1999. Discusses the global politics of nuclear proliferation following World War II. Includes the chapter “Longing for International Control, Banking on American Superiority: Harry S. Truman’s Approach to Nuclear Weapons.” Bibliographical references, index.
  • citation-type="booksimple"

    xlink:type="simple">Hales, Peter B. Atomic Spaces: Living on the Manhattan Project. Urbana: University of Illinois Press, 1997. A unique cultural history of the communities and other spaces created, and destroyed, to make way for the Manhattan Project’s progress. Looks at how the project affected local populations. A recommended study at the intersection of technology and culture. Bibliographical references, index.
  • citation-type="booksimple"

    xlink:type="simple">Hewlett, Richard G. A History of the United States Atomic Energy Commission. Vol. 1 in The New World, 1939-1946, by Richard G. Hewlett and Oscar E. Anderson, Jr. University Park: Pennsylvania State University Press, 1962. The early chapters detail the major steps that led to development of the Manhattan Project.
  • citation-type="booksimple"

    xlink:type="simple">Howes, Ruth H., and Caroline L. Herzenberg. Their Day in the Sun: Women of the Manhattan Project. Philadelphia: Temple University Press, 1999. A history of the oft-forgotten role of women in the Manhattan Project, who were critical to the program as nuclear scientists, physicists, chemists, mathematicians, biologists and medical scientists, and technicians. Bibliography, index.
  • citation-type="booksimple"

    xlink:type="simple">Jones, Vincent C. Manhattan, the Army, and the Atomic Bomb. Washington, D.C.: Center of Military History, U.S. Army, 1986. Discusses the U.S. Army’s role in the development of the nuclear bomb.
  • citation-type="booksimple"

    xlink:type="simple">Kelly, Cynthia C., ed. Oppenheimer and the Manhattan Project: Insights into J. Robert Oppenheimer, “Father of the Atomic Bomb.” Hackensack, N.J.: World Scientific, 2006. A concise but comprehensive history of Oppenheimer’s scientific legacy. Bibliography, index.
  • citation-type="booksimple"

    xlink:type="simple">_______. Remembering the Manhattan Project: Perspectives on the Making of the Atomic Bomb and Its Legacy. Hackensack, N.J.: World Scientific, 2004. A report on the proceedings of the 2002 Atomic Heritage Foundation’s Symposium on the Manhattan Project, held in Washington, D.C. Includes bibliographical references and index.
  • citation-type="booksimple"

    xlink:type="simple">Rhodes, Richard. The Making of the Atomic Bomb. New York: Simon & Schuster, 1986. Provides a wealth of scientific information and integrates it with the decision-making process that went into producing the bomb.
  • citation-type="booksimple"

    xlink:type="simple">Sherwin, Martin J. A World Destroyed: The Atomic Bomb and the Grand Alliance. New York: Alfred A. Knopf, 1975. Part 1 details major developments in physics and the critical political decisions.
  • citation-type="booksimple"

    xlink:type="simple">Stoff, Michael B., Jonathan F. Fanton, and R. Hal Williams, eds. The Manhattan Project: A Documentary Introduction to the Atomic Age. Philadelphia: Temple University Press, 1991. Contains key documents, including Einstein’s letter of August 2, 1939.

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Nuclear Non-Proliferation Treaty Goes into Effect

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