Technology & Medicine › Atomic & space age › Nuclear energy (bombs and reactors)
Manhattan Project
1942–1946· Event· Tools, energy & materials
Well documentedSolid records, physical evidence or scientific consensus.
The Manhattan Project was the secret US-led effort, with Britain and Canada, that built the first atomic bombs during World War II.
See it on the Technology & Medicine map →How it works
Driven by fear of a Nazi bomb, the US Army took charge in 1942 under General Leslie Groves, with physicist J. Robert Oppenheimer directing the Los Alamos design lab in New Mexico. Giant plants at Oak Ridge, Tennessee, separated uranium-235, and reactors at Hanford, Washington, made plutonium. It employed about 130,000 people at its peak and cost about $2 billion, roughly $30 billion today. The first test, Trinity, exploded in New Mexico on July 16, 1945.
Why it mattered
It created nuclear weapons, launched the US national-lab system and showed how government money could drive big science, a model later used for space and computing.
How we know
Declassified government records, the official Smyth Report (August 1945), Groves's and Oppenheimer's papers, and preserved sites in the Manhattan Project National Historical Park.
Still unknown or debated
Spies, including physicist Klaus Fuchs, passed secrets to the Soviet Union; historians still debate how much this sped up the 1949 Soviet bomb.
Connections (2)
▸Discovery of nuclear fission prompted the Manhattan ProjectWell documented
The connection
Within weeks of fission's announcement, physicists saw that a bomb might be possible. Fearing Germany would build one first, Leó Szilárd drafted a letter, signed by Albert Einstein, warning President Roosevelt in August 1939. Britain's 1941 MAUD report concluded a bomb needed only about 10 kilograms of uranium-235, pushing the US into a full program.
When and how
Einstein–Szilárd letter, August 2, 1939; Britain's MAUD Committee report, July 1941; full US commitment, December 1941 to mid-1942.
How we know
The Einstein letter and MAUD report are preserved and published; officials' memos show these reports drove US decisions.
Debates and open questions
How close Nazi Germany came to a bomb is still debated; most historians think its program was far behind, partly because of scientific and political misjudgments.
▸Manhattan Project built the bombs used in the Atomic bombings of Hiroshima and NagasakiWell documented
The connection
The project produced two bomb designs: a uranium 'gun' type that fired one piece of uranium-235 into another, and a plutonium implosion type, tested at Trinity in July 1945. With Germany already defeated in May, the bombs were used against Japan, one of each design in August 1945.
When and how
Built at Los Alamos, Oak Ridge and Hanford; B-29 bombers flew from Tinian in the Mariana Islands; President Truman approved their use.
How we know
Military orders, Target Committee minutes, flight records and the Smyth Report document the decision and the missions.
Debates and open questions
Some project scientists, in the June 1945 Franck Report, urged a demonstration instead of use on cities; whether such alternatives were realistic is still argued.