The Manhattan Project Builds the Atomic Bomb
A discovery in physics becomes hundreds of thousands of workers, three secret cities, and two bombs dropped on Japan
Quick facts
- Key sites
- Los Alamos, NM; Oak Ridge, TN; Hanford, WA
- Trinity test
- 16 July 1945, Alamogordo, New Mexico
- Bombings
- Hiroshima, 6 August 1945; Nagasaki, 9 August 1945
- Estimated deaths
- c. 80,000 (Hiroshima) and c. 40,000 (Nagasaki) immediate; many more from radiation
What happened
Within three years of the discovery of nuclear fission, the United States turned the physics into a weapon. The Manhattan Project, described by the National Park Service as an unprecedented, top-secret World War II government program in which the United States rushed to develop and deploy the world's first atomic weapons before Nazi Germany, employed hundreds of thousands of workers across three primary sites: a uranium enrichment complex at Oak Ridge, Tennessee, a plutonium production complex at Hanford, Washington, and a bomb design laboratory at Los Alamos, New Mexico, led by J. Robert Oppenheimer under General Leslie Groves. On 16 July 1945 the project successfully tested a plutonium device, code-named Gadget, at the Trinity site in New Mexico, an event the National Park Service says ushered in the nuclear age. On 6 August 1945 a uranium bomb called Little Boy was dropped on Hiroshima, Japan; a HISTORY-hosted account puts the immediate death toll at an estimated 80,000 people, with tens of thousands more dying later of radiation exposure. Three days later a plutonium bomb called Fat Man was dropped on Nagasaki, killing an estimated 40,000 people immediately, with combined estimates for both cities, counting later deaths from radiation, ranging from roughly 130,000 to 215,000.
Why it matters
The Manhattan Project turned a theoretical result in nuclear physics into a functioning weapon within six years, ending the Second World War in the Pacific but opening the nuclear age and the arms race that followed it. It stands as physics' starkest demonstration that a scientific discovery, however abstract its origin, carries no guarantee of how it will be used; Meitner herself, invited to join the project, refused, saying she would have nothing to do with a bomb.
How we know
The Manhattan Project's scale, sites, and outcomes are documented in declassified U.S. government records, National Park Service historical materials at the Manhattan Project National Historical Park, and casualty studies of the Hiroshima and Nagasaki bombings conducted by American and Japanese researchers in the decades since.
Sources
- U.S. National Park Service. Manhattan Project · Primary source (author-declared)nps.gov · Cited as a "primary" source (no stronger domain match). · Link is live and its text matches the event's key terms (Jul 2026)
- HISTORY (A&E Networks). Bombing of Hiroshima and Nagasaki · Reputable sourcehistory.com · The domain "history.com" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
See something wrong? . Corrections with a source get fixed fastest.
Part of a timelineHistory of Physics24 events · A search for the rules the universe obeys, from falling stones and floating crowns to a boson found underground and ripples in spacetimeView all →