Battles & Wars › World War II › Atomic bombings of Hiroshima and Nagasaki
Manhattan Project
1942–1946· Arms maker, factory or program· Arms makers & war industry
Well documentedSolid records, physical evidence or scientific consensus.
The secret US-led wartime program, with British and Canadian help, that built the first atomic bombs.
See it on the Battles & Wars map →What happened
Fears that Nazi Germany might build a bomb, raised in Einstein's 1939 letter to Roosevelt, led the US Army to take over atomic research in 1942 under General Leslie Groves, with J. Robert Oppenheimer directing the Los Alamos weapons lab. Enrico Fermi's team achieved the first chain reaction in Chicago that December. Huge plants at Oak Ridge, Tennessee, and Hanford, Washington, made bomb fuel. About 130,000 people worked on it, costing about $2 billion. The first test, Trinity, came on July 16, 1945.
Why it matters
It opened the nuclear age, helped end the war with Japan, and made nuclear weapons and deterrence central to Cold War rivalry and world politics ever since.
How we know
Declassified project records, the 1945 Smyth Report, memoirs by Groves and the scientists, preserved sites at Los Alamos, Oak Ridge and Hanford (now a national historical park), and Richard Rhodes's history.
Still debated
Most historians now agree Germany was never close to a bomb; how much Soviet spies such as Klaus Fuchs sped the 1949 Soviet test is still discussed.
Inside this (3)
Connections (5)
▸Manhattan Project built the bombs used in Atomic bombings of Hiroshima and NagasakiWell documented
The connection
The project produced two bomb designs: a uranium 'gun' bomb (Little Boy), so reliable it was never tested, and a plutonium implosion bomb (Fat Man), tested at Trinity in July 1945. Bomb parts were shipped to Tinian in the Mariana Islands, where B-29 crews loaded them for the August missions.
When and how
Los Alamos, New Mexico, to Tinian Island, July–August 1945; the 509th Composite Group under Colonel Paul Tibbets flew the missions.
How we know
Project and military records, the Target Committee minutes and Truman's statements document the chain from design to use.
Debates and open questions
Whether Truman made a clear, deliberate decision, or largely followed momentum set by the project and the military, is debated by historians.
▸Manhattan Project began the age of Nuclear weapons and deterrenceWell documented
The connection
The Manhattan Project built the first atomic bombs, proving one weapon could destroy a city. Its science, and Soviet spying on it, spread the capability: the Soviet test in 1949 ended the US monopoly and made deterrence, rather than one-sided advantage, the logic of the nuclear age.
When and how
Los Alamos, Oak Ridge and Hanford, 1942–1945; the Trinity test in New Mexico on July 16, 1945; the Soviet test in Kazakhstan on August 29, 1949.
How we know
Project records, the Smyth Report (1945), and convictions of spies such as Klaus Fuchs; Soviet archives confirm espionage sped up their bomb.
Debates and open questions
How many years espionage saved the Soviets, with estimates from one to several, and whether a US monopoly could have lasted longer, is debated.
▸Franklin D. Roosevelt authorized Manhattan ProjectWell documented
The connection
In October 1939 Roosevelt received a letter signed by Albert Einstein, drafted with physicist Leo Szilard, warning that Germany might build an atomic bomb. He set up a uranium committee, approved an all-out effort in late 1941, and in 1942 the Army took it over as the Manhattan Project, which spent about $2 billion in secret.
When and how
White House, October 11, 1939, when economist Alexander Sachs delivered the letter; full program approved October 1941; Manhattan Engineer District created August 1942.
How we know
The Einstein–Szilard letter and Roosevelt's reply survive at the FDR Library; project records were declassified after the war.
Debates and open questions
Roosevelt left little record of how he intended the bomb to be used; what he would have decided in 1945 is unknown.
▸Manhattan Project led to the far more powerful Hydrogen bombWell documented
The connection
Even during the Manhattan Project, Edward Teller pushed for a 'Super' bomb using fusion. After the Soviets tested an atomic bomb in 1949, President Truman ordered a crash program in January 1950 over the objections of Oppenheimer's advisory committee. The atomic bomb itself became the trigger that ignites the fusion fuel.
When and how
Berkeley summer study, 1942, where Teller raised the idea; Truman's decision on January 31, 1950; the Ivy Mike test, November 1952.
How we know
Declassified Atomic Energy Commission records, the General Advisory Committee's October 1949 report and participants' memoirs document the path.
Debates and open questions
Whether the US could have negotiated a mutual ban on H-bombs before testing, as some scientists urged, is a long-standing debate.
▸B-29 Superfortress program cost even more than the Manhattan ProjectWell documented
The connection
The B-29 was the costliest weapons program of World War II, roughly $3 billion against about $2 billion for the Manhattan Project. The two were linked: the bomb had to fit a B-29, and modified bombers trained for the atomic mission from 1944.
When and how
United States, 1940–1945; the Army Air Forces, Boeing and partner factories; the Manhattan Project under General Leslie Groves.
How we know
Wartime budget figures from Army and congressional records, commonly cited in histories of both programs.
Debates and open questions
Comparisons depend on what is counted, such as development, production, bases and training, so the exact gap between the two is uncertain.