The Manhattan Project: your guide to the building of the first atomic bomb

Watch: Could the Nazis have built the first atomic bomb?

Persuaded by the letter, Roosevelt authorised the establishment of the Advisory Committee on Uranium, which was later subsumed into the National Defense Research Committee (NDRC) in June 1940. Meanwhile, across the Atlantic, two Jewish émigrés from Nazi Germany to Britain – physicists Otto Frisch and Rudolf Peierls – estimated that the fissile material needed to sustain a chain reaction within a hypothetical bomb would be small enough for conveyance by aeroplane.

Albert Einstein signed a letter written by his colleague, Leo Szilard, urging US president Franklin D Roosevelt to fund new research into uranium – the heaviest naturally occurring element in the periodic table

Worried that the Nazis might draw the same conclusions, the duo sent a memorandum to the British government, making it clear that a “super-bomb” would yield an explosion powerful enough to devastate an entire city, equivalent to approximately 1,000 tonnes of dynamite. In January 1942, just over a month after Japan’s attack on Pearl Harbor and America’s entry into the Second World War, Roosevelt secretly approved plans to create the world’s first atomic bomb.

Leslie Groves of the US Army Corps of Engineers was tasked with overseeing the construction of the laboratories and factories needed to fulfil the objective, and in August, the Manhattan Project was born. Despite being named after the New York borough where its headquarters was initially located, the project’s operations quickly expanded to other US locations.

Who worked on the Manhattan Project?

A colossal endeavour, the project not only mobilised the country’s leading scientists, but countless other workers spanning a variety of expertise. At its peak, it is estimated that roughly 130,000 individuals were involved. Known as the ‘father’ of the atomic bomb, Julius Robert Oppenheimer is perhaps the project’s most famous figure today.

Briefed with weaponising atomic energy, the physicist was chosen to head up the project’s secret research laboratory in Los Alamos, New Mexico. Prior to taking up the post, he had carried out research into the quantity of radioactive material that would be required to make an atomic weapon, and how destructive it might be. Such insight proved indispensable to the project’s ultimate success.

Meanwhile, Klaus Fuchs is arguably the Manhattan Project’s most notorious individual. A German physicist and refugee to Britain, it would later transpire that he had been passing information to the Soviets throughout his involvement with the project. In fact, his espionage may have enabled the Kremlin to develop the Soviet Union’s first atomic bomb in 1949 – one year sooner than would have otherwise been the case.

Another interesting figure connected to the project is Leona Woods. Aged just 23 at its inception, she was both the youngest and the only female member of the team involved in building the world’s first-ever artificial nuclear reactor, Chicago Pile-1. She later oversaw construction of the reactors used to produce plutonium for the project.

Where was the Manhattan Project?

The quest to create an atomic bomb was a mammoth effort, with the University of Chicago and the University of California, Berkeley being among the key institutions involved. Alongside research into uranium, a new element – plutonium – had been discovered at Berkeley in 1940. Like uranium-235, scientists discovered that two of its isotopes were fissile, and it thus became central to the project’s outcome.

Besides the laboratory at Los Alamos, new weapons facilities were also created at Oak Ridge, Tennessee and Hanford, Washington, while major US corporations like Union Carbide and DuPont were tasked with building and maintaining infrastructure and machinery in coordination with the US Army. Oak Ridge’s role was to extract enough explosive component (uranium-235) to make a viable weapon, while Hanford hosted enormous reactors designed to produce plutonium. In addition, uranium – the fuel that powered the reactors – came from natural deposits located in Colorado, Canada and the Belgian Congo.

When did the first test of an atomic bomb take place?

Just before dawn on 16 July 1945, a plutonium device was detonated in the Jornada del Muerto desert, New Mexico. An incandescent flash banished the night, followed by an immense fireball that cooled into a soaring mushroom cloud, 7.5 miles tall. Gathered at a safe distance, witnesses felt the heat blast to be oven like.

The ‘Trinity’ test, as it was named, left a profound impression on its architects. Oppenheimer, reflecting some years later, said it reminded him of a verse in a sacred Hindu text, the Bhagavad Gita: “Now I am become Death, the destroyer of worlds”. The atomic age had well and truly begun.

What were the consequences of the Manhattan Project?

Days before the Potsdam Declaration (an Allied ultimatum demanding Japan’s surrender), 70 Manhattan Project scientists petitioned the new US president Harry Truman to consider the “obligation of restraint” such weaponry elicited. They reminded him that Germany’s defeat rendered the original purpose behind the bomb’s creation obsolete, but their letter – dated 17 July 1945 – never reached him.

On 6 August, Enola Gay, a B-29 bomber, dropped ‘Little Boy’ – an untested uranium-235 device – over the Japanese city of Hiroshima, which exploded 580 metres above the ground. Around 70,000 people were killed outright, with the resulting radiation claiming lives for decades afterwards. Two-thirds of the city was destroyed instantly. Three days later, ‘Fat Man’, a duplicate of the Trinity bomb, exploded over Nagasaki, killing between 60,000 and 80,000 people. Japan surrendered on 15 August, and World War II formally came to an end on 2 September.

The subsequent Cold War was defined by a nuclear arms race between the US and the Soviet Union, leading to a policy of mutual assured destruction (MAD) and a worldwide peace movement calling for nuclear disarmament. Nearly 80 years since their destruction, debate still rages around the morality of targeting Hiroshima and Nagasaki, as well as the risk that nuclear weapons continue to pose to this day.

This article was first published in the January 2023 issue of BBC History Revealed