Manhattan Project

Under the codename Manhattan, American, British and Canadian scientists and engineers secretly built the first atomic bombs during the Second World War. The project, run mainly by the United States, was directed from 1942 to 1946 by General Leslie Groves of the US Army Corps of Engineers, while the physicist J. Robert Oppenheimer led the laboratory at Los Alamos that designed the weapons. At its peak it employed nearly 130,000 people and it cost close to 2 billion US dollars. Its bombs destroyed Hiroshima and Nagasaki in August 1945 and opened the nuclear age.

Nuclear fission, found by Otto Hahn and Fritz Strassmann in 1938 and interpreted by Lise Meitner and Otto Frisch, made a bomb theoretically possible, and scientists feared Germany might build one first. Leo Szilard and Eugene Wigner wrote a letter in August 1939, which Albert Einstein signed before it went to President Franklin D. Roosevelt as a warning. American research at first moved slowly. In Britain, Frisch and Rudolf Peierls estimated that a bomb might need only around 10 kilograms of uranium-235; their memorandum of March 1940 launched a British project, whose MAUD Committee recommended building one. Its findings, pressed on American physicists by Mark Oliphant in 1941, helped persuade Roosevelt on 9 October 1941 to speed up the work. In June 1942 he approved a large programme under the army.

The army's part was named the Manhattan District, after its first offices in New York, and it was formally created on 13 August 1942. Groves took command on 23 September 1942 and chose Oppenheimer to lead the weapons laboratory, known as Project Y, despite doubts about his lack of administrative experience. Britain's own project, Tube Alloys, fell behind, and the Quebec Agreement between Roosevelt and Winston Churchill in August 1943 brought British scientists into the American effort.

Most of the money went on vast plants to produce fissile material. Enrico Fermi's team produced the first man-made chain reaction able to sustain itself on 2 December 1942, in a reactor in Chicago known as Chicago Pile-1. At Oak Ridge in Tennessee, where about 1,000 families were moved off their land, plants enriched uranium; the town's population reached 75,000 in May 1945. At Hanford in Washington state, the world's first industrial-scale reactors made plutonium. Because Hanford plutonium proved unsuitable for a simple gun-type weapon, Los Alamos also developed a more complex implosion design.

The first nuclear explosion took place at 05:30 on 16 July 1945 in the New Mexico desert, when the Trinity test released energy equal to roughly 20 kilotons of TNT. President Harry Truman, then at the Potsdam Conference, told Stalin only that the United States had a powerful new weapon; Soviet spies had in fact kept Stalin informed.

On 6 August 1945 a uranium bomb, Little Boy, was dropped on Hiroshima. Early estimates put the dead at 66,000, while later estimates suggest up to 140,000 had died by December 1945. On 9 August a plutonium bomb, Fat Man, was dropped on Nagasaki after cloud hid the primary target, Kokura, killing tens of thousands more. On 15 August Japan announced that it would surrender. Whether the atomic bombings were necessary to end the war remains debated by historians, and some project scientists had urged a demonstration first.

The project also gathered intelligence on Germany's nuclear programme through the Alsos mission. Despite strict security, it was penetrated by Soviet agents, the most important being the physicist Klaus Fuchs, whose espionage was uncovered only in 1950.

After the war the project carried out tests at Bikini Atoll and laid the foundations of the national laboratories. Control passed to the civilian Atomic Energy Commission on 1 January 1947.

Our 1938 and 1945 maps show the United States and the Empire of Japan before and at the end of the war in the Pacific.