The Manhattan Project was a secret research and development program that produced the first nuclear weapons during World War II. Led by the United States with British and Canadian participation, it brought together military administrators, scientists, industrial contractors, and a vast workforce. Its principal wartime phase began in 1942; its scientific and industrial facilities passed to civilian control in January 1947. The project developed the weapons tested at Trinity and used against Hiroshima and Nagasaki in August 1945. (osti.gov)
Scientific and political origins
The project emerged from the discovery of nuclear fission in late 1938. Otto Hahn and Fritz Strassmann obtained experimental evidence that uranium nuclei could split; Lise Meitner and Otto Frisch supplied the physical interpretation. Fission released energy and additional neutrons, raising the possibility of a self-sustaining nuclear chain reaction. Scientists recognized that this process might support either controlled energy production or an explosive weapon. (osti.gov)
Concern that Nazi Germany might develop such weapons encouraged American government involvement. In August 1939, Albert Einstein signed a letter, prepared with physicist Leo Szilard, warning President Franklin D. Roosevelt about the military possibilities of uranium research. Roosevelt established an advisory uranium committee that October. Initially modest research expanded through successive civilian organizations, particularly as evidence accumulated that an atomic bomb was technically feasible. The letter was an early stimulus, not the immediate creation of the full-scale project. (osti.gov)
Organization and international cooperation
In August 1942, the Army Corps of Engineers established the Manhattan Engineer District to manage the expanding program. Its name reflected its original headquarters in Manhattan, New York, rather than the location of its principal laboratories and factories. Leslie R. Groves assumed overall leadership in September, directing construction, procurement, security, and coordination between scientific and industrial operations. (nps.gov)
Groves selected J. Robert Oppenheimer to lead the weapons laboratory at Los Alamos. The program connected university research with industrial engineering on an exceptional scale. According to the Department of Energy’s historical overview, employment reached approximately 130,000 at its peak, and expenditure totaled about $2.2 billion by the end of the war, in contemporary dollars. (osti.gov)
British research contributed important early assessments of bomb feasibility. The Quebec Agreement, signed by Roosevelt and Winston Churchill on August 19, 1943, established the framework for Anglo-American cooperation. Canadian participation included research, materials, and representation in the arrangements coordinating the allied effort. (osti.gov)
Scientific and industrial centers
The project pursued two routes to fissile material: separating uranium-235 from natural uranium and producing plutonium in reactors. At the University of Chicago, a team led by Enrico Fermi achieved the first controlled, self-sustaining nuclear chain reaction on December 2, 1942, in Chicago Pile-1. This demonstrated a fundamental requirement for reactor-based plutonium production. (home.nps.gov)
Three locations became the principal centers:
- Oak Ridge, Tennessee: Plants concentrated uranium-235, a relatively scarce isotope of natural uranium. Electromagnetic separation, gaseous diffusion, and thermal diffusion were developed as complementary industrial processes.
- Hanford, Washington: Production reactors and chemical separation facilities manufactured plutonium. The B Reactor was the first full-scale plutonium-production reactor.
- Los Alamos, New Mexico: Scientists and engineers designed, assembled, and evaluated the weapons, integrating theoretical calculations with experimental work. (osti.gov)
These centers depended on a wider network of laboratories, factories, suppliers, and raw-material sources. Uranium procurement extended beyond the United States, notably to Canada and the Belgian Congo, making the project an international undertaking despite the concentration of its largest installations on American territory. (osti.gov)
Testing and wartime use
Los Alamos developed a uranium weapon using the gun-assembly principle and a plutonium weapon using implosion. Reactor-produced plutonium proved unsuitable for the planned gun-type design, making implosion a central technical challenge. Confidence in the uranium design was sufficient that it was not subjected to a full nuclear explosive test before use. The plutonium design was tested at Trinity in southern New Mexico on July 16, 1945—the first nuclear explosion. (nps.gov)
On August 6, the United States dropped the uranium bomb Little Boy on Hiroshima. On August 9, it dropped the plutonium bomb Fat Man on Nagasaki. Both caused extensive destruction and killed and injured tens of thousands of people. Japan announced its surrender on August 15 and formally signed the surrender instrument on September 2, 1945. (home.nps.gov)
Secrecy and social consequences
Security restricted information to personnel who needed it, leaving many workers unaware of the project’s ultimate purpose. Nevertheless, espionage penetrated the program: physicist Klaus Fuchs transmitted nuclear information to the Soviet Union. (govinfo.gov)
Construction also displaced existing communities. Around Oak Ridge, roughly 3,000 residents were removed from land acquired for the installations. African American workers contributed substantially, particularly at Oak Ridge and Hanford, while encountering segregated housing, facilities, and employment practices. Trinity deposited radioactive fallout across inhabited areas of New Mexico, extending the project’s consequences beyond its secured sites. (nps.gov)
Postwar institutions
The Atomic Energy Act of 1946 established the United States Atomic Energy Commission, which assumed control of the project’s facilities on January 1, 1947. Its laboratories and production infrastructure supported subsequent weapons development, nuclear power, and scientific research. In 2015, the Manhattan Project National Historical Park was established to preserve and interpret sites at Oak Ridge, Hanford, and Los Alamos. (energy.gov)