The Manhattan Project: How the US built the bomb
The bomb that destroyed Hiroshima on August 6, 1945 was the result of a top-secret programme dubbed the Manhattan Project, one of the biggest scientific and industrial challenges in history.
The bomb was built by a group of scientists, including European refugees, who began in 1942 and raced against time in the belief that Nazi Germany was also trying to make one.
Secrecy was such that vice president Harry Truman, who would eventually decide to use the weapon, was unaware of the project for several years.
In late 1938 German chemist Otto Hahn discovered the principle of nuclear fission while working at the Kaiser Wilhelm Institute in Berlin.
Albert Einstein, who had sought asylum in the United States five years earlier, signed a letter to then-president Franklin Roosevelt to warn that it could be possible to build "extremely powerful bombs of a new type".
Roosevelt created a research and development body but much of the early work took place in Britain, which faced a far greater threat at the time from the Nazi war machine.
Given the scientific and industrial resources available in the United States, along with the geographic protection it afforded, development shifted back to North America and in June 1942 the US Army was given the task of building the laboratories and plants needed to make a bomb.
The Manhattan Project was placed under the command of General Leslie Groves of the US Army Corps of Engineers.
It was a huge task: each of the potential solutions for achieving a nuclear explosion presented industrial challenges and the amount of money spent -- more than two billion dollars at the time -- was an expensive bet for the United States and Britain.
- Light more intense than the sun -
Groves chose Robert Oppenheimer to direct the project at a site in Los Alamos, New Mexico.
In Chicago, meanwhile, Italian-born physicist Enrico Fermi, the 1938 Nobel laureate, created the first controlled nuclear chain reaction.
In Oak Ridge, Tennessee, thousands of workers built reactors to produce uranium 235 fuel for the bomb.
Meanwhile, physicist Niels Bohr, another Nobel prize winner, fled from his occupied native Denmark in 1943 and joined the team in Los Alamos, bringing decisive expertise to the project.
Once the theoretical issues had been resolved, the scientists had to solve technical problems such as how to trigger an explosion. Another hurdle was that of producing a bomb that could be carried and launched with precision by an aeroplane.
Progress nonetheless continued and in mid-1944 Groves chose a site near Alamogordo, New Mexico, for a test.
Less than a year later, three bombs were ready and one dubbed "The Gadget" was set up in the desert.
The others -- "Little Boy", which used uranium 235 as fuel, and "Fat Man", which used plutonium -- were readied for shipment to the Pacific.
The desert test -- the world's first atomic bomb explosion -- took place on July 16, 1945 and was dubbed "Trinity" by Oppenheimer. Scientists and other witnesses were housed in bunkers, some of which were just five miles (eight kilometres) from the blast's epicentre.
"The whole country was lighted by a searing light with the intensity many times that of the midday sun. It was golden, purple, violet, grey, and blue. It lighted every peak, crevasse and ridge of the nearby mountain range with a clarity and beauty that cannot be described but must be seen to be imagined," General Thomas Farrell wrote later.
The news reached Truman in Potsdam, Germany where he was meeting Joseph Stalin and Winston Churchill.
"The test was successful beyond the most optimistic expectations of anyone," Groves wrote.
With the death of Roosevelt, it fell to Truman, the next president, to decide whether to use the bomb on Japan, which he did two weeks later.
The Manhattan Project allowed the United States to gain the strategic upper hand and to end World War II.
But in deciding to bomb Hiroshima, Truman unleashed a force that the world has struggled to control ever since.