The Manhattan Project was the codename for the American-led effort to build atomic weapons during the Second World War. Launched in 1942 and run under extraordinary secrecy, it employed more than 130,000 people at a cost of roughly $2 billion in 1940s dollars (about $30 billion in present terms). It ended a war, began an arms race, and forced physicists, generals, and presidents to confront a new kind of power.
Origins
The scientific foundation was laid in 1938, when Otto Hahn and Fritz Strassmann demonstrated nuclear fission in Berlin, and Lise Meitner and Otto Frisch, working in neutral Sweden, gave the first theoretical account. By the summer of 1939, émigré physicists in the United States understood that a fission bomb was feasible. In August 1939, Leo Szilard drafted a letter to President Franklin Roosevelt warning of the possibility; Albert Einstein signed it. Delivered in October 1939, it produced the Advisory Committee on Uranium, which became the S-1 Committee, and a small research budget.
The decisive push came after the British MAUD Report of July 1941, which judged a 10-kiloton bomb feasible by 1944. On December 6, 1941 — one day before Pearl Harbor — the government committed to an all-out effort. In June 1942, the Army Corps of Engineers took over the program, and in September 1942, Brigadier General Leslie Groves was named director.
Organization and scope
Groves built the project on a wartime-industrial model. J. Robert Oppenheimer, a Berkeley theoretical physicist with no managerial experience, was chosen to run the weapons-design laboratory at Los Alamos, New Mexico, a site that did not exist on any map before 1943. Production was distributed: gaseous-diffusion and electromagnetic-isotope uranium enrichment at Oak Ridge, Tennessee; plutonium production in graphite reactors at Hanford, Washington; heavy-water work in Chicago; and British-linked research at Montreal and Birmingham.
The workforce peaked at roughly 130,000. About 600,000 Americans were ultimately employed in activities related to the project. Secrecy was enforced through compartmentalization, military censorship, and a publicity program that concealed even the scale of the Hanford and Oak Ridge sites from the local populations that built them.
The scientists and their doubts
Los Alamos gathered a remarkable community: Oppenheimer, Enrico Fermi, Edward Teller, Hans Bethe, Emilio Segrè, Glenn Seaborg, John von Neumann, and a generation of younger theorists and engineers. Many were refugees from fascist Europe. The atmosphere combined wartime urgency, brilliant improvisation, and a moral disquiet that grew as the weapon’s power became clear. In June 1945, a group led by James Franck warned Secretary of War Henry Stimson that a demonstration detonation would be politically preferable to combat use. The Franck Report was set aside. Leo Szilard’s petition, signed by 70 Manhattan Project scientists and delivered to Truman in July 1945 urging that the bomb not be used, was never opened.
Trinity, July 16, 1945
The first detonation took place at 5:29 a.m. on July 16, 1945, on the Jornada del Muerto in central New Mexico. The plutonium implosion device, nicknamed “the Gadget,” was hoisted to the top of a 100-foot steel tower. The yield was about 21 kilotons. The blast fused desert sand into a green glass later called trinitite. Standing at base camp, Oppenheimer later said, recalled a line from the Bhagavad Gita: “Now I am become Death, the destroyer of worlds.” The blast was felt 100 miles away and visible 200 miles distant. Kenneth Bainbridge, the test director, told Oppenheimer at the moment of detonation, “Now we are all sons of bitches.”
Hiroshima and Nagasaki
The combat use of atomic weapons remains the most contested decision of the modern era. On July 26, 1945, the Allies issued the Potsdam Declaration, warning Japan to surrender unconditionally or face “prompt and utter destruction.” On August 6, a B-29 piloted by Paul Tibbets dropped a uranium-235 gun-type bomb, “Little Boy,” on Hiroshima. The yield was about 15 kilotons; immediate deaths were 70,000–80,000; total deaths by year’s end approached 200,000. On August 9, a B-29 dropped a plutonium implosion bomb, “Fat Man,” on Nagasaki. The yield was about 21 kilotons; immediate deaths 40,000; total by year’s end perhaps 140,000. Japan announced surrender on August 15; the formal instrument was signed aboard the USS Missouri on September 2.
Ethics and aftermath
The decision to use the bomb has been defended as the alternative to an invasion of the home islands, projected at the time to cost hundreds of thousands of American casualties, and as the shortest path to ending a war that had already killed millions. Critics counter that Japan was near collapse, that a demonstration explosion was feasible, that Soviet entry into the Pacific war on August 8 may have done more to force surrender than the second bomb, and that targeting civilians violated the emerging norms of distinction and proportionality that the bomb itself would later help to entrench. The argument has not been settled, and probably cannot be. The Manhattan Project remains the central case study in the moral responsibility of science.
Key dates, 1939–1945
| Date | Event |
|---|---|
| 1939-08-02 | Einstein–Szilárd letter delivered to President Roosevelt warning of a possible German atomic bomb. |
| 1941-12-06 | Roosevelt commits the United States to an all-out effort to build an atomic weapon. |
| 1942-11-25 | General Groves selects Los Alamos, New Mexico, as the weapons-design laboratory site. |
| 1942-12-02 | First self-sustaining nuclear chain reaction achieved at Chicago Pile-1. |
| 1944-09-26 | Hanford B reactor in Washington state produces its first plutonium. |
| 1945-07-16 | Trinity test at Alamogordo, New Mexico — first detonation of a nuclear weapon (~21 kt). |
| 1945-08-06 | ”Little Boy” dropped on Hiroshima, Japan (~15 kt). |
| 1945-08-09 | ”Fat Man” dropped on Nagasaki, Japan (~21 kt). |
Key sources
- Richard Rhodes, The Making of the Atomic Bomb (Simon & Schuster, 1986)
- Kai Bird and Martin J. Sherwin, American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer (Knopf, 2005)
- Richard G. Hewlett and Oscar E. Anderson Jr., A History of the United States Atomic Energy Commission, Vol. 1 (Pennsylvania State University Press, 1962)
- Gar Alperovitz, The Decision to Use the Atomic Bomb and the Architecture of an American Myth (Knopf, 1995)