'Manhattan' was the Allied extraordinarily secret undertaking in the USA to develop the atomic bomb, first tested with complete success at Alamogordo in the New Mexico desert in July 1945 (autumn 1942/January 1947).

The 'Manhattan' Project or, more formally, the Manhattan Engineering District (MED), was undertaken by US scientists with considerable assistance from British and Canadian scientists. The research aspect of the programme was directed by a US physicist, Dr J. Robert Oppenheimer, and the whole project was supervised by Major General Leslie R. Groves under the auspices of the US Army’s Corps of Engineers.

The origins of the programme can be found in the fears that emerged in the 1930s from the revelations that Germany was undertaking research into the possibility of creating nuclear weapons. The 'Manhattan' Project assumed control over all US fission research, which had begun in 1939, from the autumn of 1942, and resulted in the design, production, and detonation of three nuclear devices in 1945. The first, using plutonium made at the Hanford plant in Washington state, was tested on 16 July in 'Trinity', the world’s first nuclear weapons test, near Alamogordo in the state of New Mexico. The second was an enriched uranium bomb codenamed 'Little Boy', which was detonated on 6 August over the Japanese city of Hiroshima. The third was another plutonium bomb, codenamed 'Fat Man', and was detonated on 9 August over the Japanese city of Nagasaki.

The MED maintained control over US nuclear weapons production until the formation of the Atomic Energy Commission in January 1947, and at its peak in 1945 the MED employed more than 130,000 persons, and cost a total of nearly US$2 billion.

After the discovery of the electron by J. J. Thomson and radioactivity by Henri Becquerel at the start of the 20^th century, the atom was no longer thought to be indivisible. In 1905 Albert Einstein showed in his Theory of Special Relativity that a small amount of mass could be converted into a large amount of energy, though the practical significance of his E=mc² equation was not fully appreciated for many years. In 1911 Ernest Rutherford demonstrated that the majority of an atom’s mass was in a small nucleus, made up of protons and surrounded by a cloud of electrons. In the 1920s, quantum mechanics provided an explanation for processes in the nucleus such as radioactive decay. In 1932, James Chadwick discovered that the nucleus contained another fundamental particle, the neutron, and in the same year John Cockcroft and Ernest Walton first 'split the atom'. In 1934, Irène and Frédéric Joliot-Curie discovered that artificial radioactivity could be induced in stable elements by bombarding them with alpha particles, and in the same year Enrico Fermi reported similar results when bombarding uranium with neutrons. In December 1938, the Germans Otto Hahn and Fritz Strassman published experimental results about the bombardment of uranium with neutrons. Collaborator Lise Meitner (a political refugee in Sweden at the time) and Otto Robert Frisch correctly interpreted these results as the splitting of the uranium nucleus after the absorption of a neutron (nuclear fission, in fact), which released a large amount of energy and additional neutrons.

In 1933 the Hungarian physicist Leó Szilárd had proposed that if a neutron-driven process released more neutrons than those required to start it, an expanding nuclear chain reaction might result. Upon experimentation, he found that the fission of uranium indeed released, on average, two or more neutrons. That such mechanisms might have implications for civilian power or military weapons was perceived by a number of scientists in many different countries at about the same time.

While all of these developments in science were taking place, however, Europe was undergoing a profound political upheaval. The most far-reaching of these was the appointment of Adolf Hitler as Chancellor of Germany in January 1933. Hitler’s anti-Semitic ideology caused all Jewish civil servants, which included many physicists at universities, to be expelled from their posts. As a result many European physicists who would later make key discoveries went into exile in the UK and USA.

After Germany invaded Poland in 1939 and so triggered World War II, many scientists in the UK and USA became anxious about the possibility of German developments in nuclear technology. Szilárd, Eugene Teller and Eugene Wigner, all of them Hungarian Jewish refugees, believed that the energy released in nuclear fission might be used in bombs by the Germans. They persuaded Einstein, one of the world’s most eminent physicists and himself Jewish, to warn President Franklin D. Roosevelt of this danger in a letter of 2 August 1939, which Szilárd drafted. In response to the warning, Roosevelt encouraged further research into the national security implications of nuclear fission.

The US Navy awarded the first atomic energy funding of $6,000 for graphite to be use in nuclear experiments. Roosevelt created an ad hoc Uranium Committee under the chairmanship of Lyman Brigs, the chief of the National Bureau of Standards. This committee began small research programmes in 1939 at the Naval Research Laboratory in Washington, DC, where physicist Philip Abelson explored uranium isotope separation. At Columbia University, Fermi, who had emigrated because his wife was Jewish, built prototype nuclear reactors using various configurations of graphite and uranium.

The Uranium Committee came under the aegis of the National Defense Research Committee in 1940. It had been thought that an atomic bomb would need tons of uranium and so would be difficult to transport. The in March 1940, in the British city of Birmingham, two more German émigrés, Otto Frisch and Rudolf Peierls, calculated that an atomic weapon needed only a few pounds of uranium-235 and so it might be practicable. They sent their report to Henry Tizard, chairman of the Committee for the Scientific Survey of Air Warfare, the most important scientific committee in the British war effort.

Tizard established a sub-committee, the MAUD Committee, to investigate the feasibility in greater depth. The chairman of this committee was G. P. Thomson, professor of physics at Imperial College, London. After commissioning further research, the MAUD Committee produced its first report in March 1941, and confirmed that a uranium bomb could be produced using 25 lb (11.3 kg) of U-235, a fissionable isotope. From this would come an explosion equivalent to that of 1,800 tons of TNT, the benchmark conventional explosive. Furthermore, the MAUD Committee’s research had shown that isotopic separation of the required quantity of U-235 was feasible. Detailed costings followed in another report of July 1941.

Meanwhile in the USA the Uranium Committee had not made comparable progress. The first MAUD Report was sent from the UK to the USA in March 1941, but no comment was received from the USA. A member of the MAUD Committee and Frisch’s and Peierls’s professor, Mark Oliphant, flew to the USA in August 1941 to find out what was being done with the MAUD reports. He found that Lyman Briggs had simply locked them in his safe, telling nobody, not even the Uranium Committee, because the USA was 'not at war'. There was little urgency elsewhere until Oliphant visited Ernest Lawrence, James Conant (chairman of the NDRC) and Fermi, and told them of the MAUD Report. Lawrence also contacted Conant and Arthur Compton, a physicist and Nobel laureate at the University of Chicago, convincing them that they should take Frisch’s and Peierls’s work very seriously.

The National Academy of Sciences then proposed an all-out effort to build nuclear weapons. On 9 October 1941 Roosevelt authorised atomic weapon development. On 6 December 1941 Vannevar Bush created a special group, the S-1 Committee, to guide the effort.

In 1941 the NDRC was subsumed into the Office of Scientific Research and Development to expand these efforts. In December 1941 scientists at the University of Chicago Metallurgical Laboratory, the University of California Radiation Laboratory and the physics department of Columbia University accelerated their efforts to prepare the nuclear materials for a weapon. Arthur Compton organised the Metallurgical Laboratory at the University of Chicago in early 1942 to study plutonium and fission piles. Compton asked Oppenheimer, a theoretical physicist of the University of California, to take over research on fast neutron calculations, which was essential to the development of a nuclear weapon. John Manley, a physicist at the University of Chicago Metallurgical Laboratory, was assigned to assist Oppenheimer find answers by co-ordinating and contacting several experimental physics groups scattered across the USA.

Plutonium seemed to provide a better method of making a bomb, but much less was known about it. Two parallel and completely separate efforts were therefore undertaken. One project produced a uranium bomb and the other route produced two plutonium bombs, all of which were successfully detonated in 1945.

During the spring of 1942, Oppenheimer and Robert Serber of the University of Illinois worked on the problems of neutron diffusion to establish how neutrons move in the chain reaction, and hydrodynamics to establish how the explosion produced by the chain reaction behaves. To review this work and the general theory of fission reactions, Oppenheimer convened a summer study at the University of California, Berkeley during June 1942. At this the theoretical physicists Hans Bethe, John Van Vleck, Edward Teller, Felix Bloch, Emil Konopinski, Robert Serber, Stanley S. Frankel and Eldred C. Nelson (the last three former students of Oppenheimer) confirmed that a fission bomb was feasible. The scientists suggested that such a reaction could be initiated by assembling a critical mass (the quantity of nuclear explosive adequate to sustain it) either by firing two sub-critical masses of plutonium or U-235 together or by imploding a hollow sphere made of these materials with a blanket of high explosives. Until more quantitative data on fission reactions became available, this was all that could be done.

Teller saw another possibility: by surrounding a fission bomb with deuterium and tritium, a much more powerful 'superbomb', which he called simply the 'Super', might be constructed. This concept was based on studies of energy production in stars made by Bethe before the war. When the detonation wave from the fission bomb moved through the mixture of deuterium and tritium nuclei, these would fuse together to produce much more energy than fission alone could achieve. This process of nuclear fusion is similar to the way that elements fuse in stars to produce light. Bethe was sceptical. As Teller pushed hard for his 'superbomb', proposing scheme after scheme, Bethe refuted each one. The fusion idea had to be put aside while the fission bombs, and the war, were completed. (The 'super', or thermonuclear device, was produced after the war and tested in 1952, after an acrimonious political fight pitting Teller against Oppenheimer, leading to loss of Oppenheimer’s official status. However the H-bomb used methods different than Teller’s, which Bethe had correctly refuted.)

Teller also raised the speculative possibility that an atomic bomb might 'ignite' the atmosphere as the result of a hypothetical fusion reaction of nitrogen nuclei. Bethe calculated, according to Serber, that it could not happen. A refutation was written by Konopinski, C. Marvin and Teller, showing that ignition of the atmosphere was impossible not just unlikely. It is believed that Oppenheimer unfortunately mentioned this to Arthur Compton, who 'didn’t have enough sense to shut up about it. It somehow got into a document that went to Washington' which led to the question 'never [being] laid to rest'. The concern was not finally laid to rest until the 'Trinity' test.

The conferences in the summer of 1942 provided the detailed theoretical basis for the design of the atomic bomb at Los Alamos. Although it involved over 30 different research and production sites, the 'Manhattan' Project was largely carried out in three secret scientific cities that were established by power of eminent domain: Hanford in Washington state, Los Alamos in New Mexico and Oak ridge in Tennessee.

The Los Alamos National Laboratory was built on a mesa that previously been the location of the Los Alamos Ranch School. The Hanford site, which grew to almost 1,000 sq miles (2590 km²), took over irrigated farm land, fruit orchards, a railway, and two active farming communities, Hanford and White Bluffs. The Oak Ridge facilities covered more than 60,000 acres (243 km²) of several former farm communities. Some Tennessee families were given notice of just two weeks to vacate family farm lands that had been their home for generations.

The existence of these sites and the secret cities of Los Alamos, Oak Ridge, and Hanford were officially secret until the end of World War II. Major 'Manhattan' Project sites and subdivisions included: Site W (Hanford) as a plutonium production facility, Site X (Oak Ridge) for enriched uranium production and plutonium production research (and also including X-10 as its graphite reactor research pilot plant, Y-12 as its electromagnetic separation uranium plant, K-25 as its gaseous diffusion separation uranium plant and S-50 as its thermal diffusion separation uranium plant), Site Y (Los Alamos) as the bomb research laboratory, the Metallurgical Laboratory (University of Chicago) for development, Project Alberta (Wendower in Utah and Tinian in the Mariana islands group) for preparing the combat delivery of the bombs, Project Ames (Ames in Iowa) for the production of raw uranium metal, Project Camel (Inyokern in California) for high-explosives research and non-nuclear engineering for the 'Fat Man' bomb, Project 'Trinity' (Alamogordo in New Mexico) for preparing the test of the first atomic device, and the Radiation Laboratory (University of California) for electromagnetic separation enrichment research.

The measurements of the interactions of fast neutrons with the materials in a bomb were essential because the number of neutrons produced in the fission of uranium and plutonium must be known, and because the substance surrounding the nuclear material must have the ability to reflect, or scatter, neutrons back into the chain reaction before it is blown apart in order to increase the energy produced. Therefore, the neutron scattering properties of materials had to be measured to find the best reflectors. Estimating the explosive power required knowledge of many other nuclear properties, including the cross section (a measure of the probability of an encounter between particles that result in a specified effect) for nuclear processes of neutrons in uranium and other elements. Fast neutrons could only be produced in particle accelerators, which were still relatively uncommon instruments in physics departments in 1942.

The need for better co-ordination was clear. By September 1942, the difficulties involved with conducting preliminary studies on nuclear weapons at universities scattered throughout the USA indicated the need for a laboratory dedicated solely to that purpose. An even greater need was the construction of massive industrial plants to produce U-235 and plutonium, the fissionable materials that would provide the nuclear explosives. Bush, the head of the civilian Office of Scientific Research and Development, asked Roosevelt to assign the large-scale operations connected with the quickly growing nuclear weapons project to the military. Roosevelt selected the US Army to work with the OSRD in building production plants. The US Army Corps of Engineers selected Colonel James Marshall to oversee the construction of factories to separate uranium isotopes and manufacture plutonium for the bomb. Marshall and his deputy, Colonel Kenneth Nichols, had to struggle to understand the various proposed processes and the scientists with whom they had to work. Thrust suddenly into the new field of nuclear physics, they felt unable to distinguish between technical and personal preferences. Although they decided that a site near Knoxville would be suitable for the first production plant, they did not know how large the site had to be and so put off its acquisition.

There were other problems. Because of its experimental nature, the nuclear weapons work could not compete with the US Army’s more urgent tasks for top-priority ratings.The selection of scientists' work and production plant construction were often delayed by Marshall’s inability to get the critical materials, such as steel, which were also needed in other military productions. Even selecting a name for the new US Army project was difficult. The title chosen by General Brehon Somervell, namely 'Development of Substitute Materials' was rejected as it seemed to reveal too much.

In the summer of 1942 Colonel Leslie Groves was deputy to the chief of construction for the Corps of Engineers and had overseen construction of the Pentagon, the world’s largest office building. Hoping for an overseas command, Groves vigorously objected when Somervell appointed him to take charge of the weapons project. His objections were overruled and Groves resigned himself to leading a project he thought had little chance of succeeding. Groves appointed Oppenheimer as the project’s scientific director, to the surprise of many as Oppenheimer’s radical political views were thought to pose security problems.

The first thing done was a renaming of the project as The Manhattan District. The name evolved from the Corps of Engineers practice of naming districts after its headquarters' city, and Marshall’s headquarters were in New York City. At the same time, Groves was promoted to brigadier general, which gave him the rank thought necessary to deal with the senior scientists in the project. Within a week of his appointment, Groves had solved the 'Manhattan' Project’s most urgent problems. His forceful and effective manner soon became all too familiar to the atomic scientists.

The first major scientific hurdle of the project was solved on 2 December 1942 under the bleachers at Staff Field at the University of Chicago, where a team led by Fermi initiated the first self-sustaining nuclear chain reaction in an experimental reactor named Chicago Pile-1. A coded phone call from Compton saying 'The Italian navigator [Fermi] has landed in the new world, the natives are friendly' to Conant in Washington, DC, brought the news that the experiment was a success.

This was a major turning point. 'Little Boy', the Hiroshima bomb, was made from U-235, a rare uranium isotope which has to be separated physically from more prevalent U-238 isotope, itself unsuitable for use in an explosive device. However U-235 is only 0.7% of raw uranium and chemically identical to the 99.3% of U-238, so various physical methods were considered for separation. One method of separating U-235 from raw uranium ore was devised by Franz Simon and Nicholas Kurti, two more Jewish émigrés, at the University of Oxford in the UK. Their gaseous diffusion method was scaled up in large separation plants at Oak Ridge Laboratories. These used uranium hexafluoride gas as the process fluid, and the method eventually produced most of the U-235.

Another method was electromagnetic isotope separation, developed by Ernest Lawrence at the University of California Radiation Laboratory at Berkeley. This method resulted in devices known as calutrons, which were effectively mass spectrometers. Initially the method seemed promising for large-scale production, but it proved to be expensive, could not produce enough material and was later abandoned. Other techniques, such as thermal diffusion, were also trialled.

The uranium bomb was a gun-type fission weapon: a critical mass of U-235 is assembled by firing one mass of U-235, the 'bullet', down a more or less conventional gun barrel into another mass of U-235, rapidly creating a critical mass of U-235 and resulting in a huge explosion. In contrast, the bombs used in the first test at the Trinity Site, New Mexico, and also in the 'Fat Man' bomb dropped over Nagasaki, were made primarily of Pu-239.

Plutonium is a synthetic element. Although U-238 is useless as fissile material for an atomic bomb, it is used to produce plutonium. The fission of U-235 produces relatively slow neutrons which are absorbed by U-238, which after a few days of decay turns into Pu-239.

The production and purification of plutonium used techniques developed in part by Glenn Seaborg while working at Berkeley and Chicago. Beginning in 1943, huge plants were built to produce plutonium at Site W outside Richland in the state of Washington.

In 1943/44 development efforts were centred on the creation of the 'Thin Man' gun-type fission bomb using Pu-239. Once this has been achieved, the 'Little Boy' U-238 version would be a relatively simple adaptation, it was thought. Initial tests of the properties of plutonium were done using cyclotron-generated Pu-239, which was very pure but could be created only in very small quantities. On 5 April 1944 Emilio Segrè at Los Alamos received the first sample of Hanford-produced plutonium. Within 10 days, he discovered a fatal flaw: reactor-bred plutonium was far less pure than cyclotron-produced plutonium, and as a result had a much higher spontaneous fission rate than U-235. The implications of this made a 'gun' detonation mechanism unsuitable: because of the relatively slow speed of the gun mechanism, a plutonium bomb would 'fizzle' (i.e. blow itself apart before it developed a substantial chain reaction). In July 1944, therefore, the difficult decision was made to cease work on the plutonium gun method: there would be no 'Thin Man'. The gun method was further developed for uranium only and, as expected, this gave few complications.

Most efforts were now directed to a different method for plutonium. Ideas of using alternative detonation schemes had existed for some time at Los Alamos. One of the more innovative had been the idea of 'implosion' whereby a sub-critical sphere of fissile material could, using chemical explosives, be forced to collapse in on itself, creating a very dense critical mass. Initially it had been entertained as a possible though unlikely method. But after it was discovered that it was the only possible solution for using plutonium in a nuclear weapon, it received the highest project priority.

By the end of July 1944, the entire project had been reorganised around solving the implosion problem, whose solution was eventually found in the use of shaped charges with many explosive lenses in order to produce the perfectly spherical explosive wave needed for proper compression of the plutonium sphere. Because of the complexity of detonating an implosion-style weapon necessary for the plutonium bomb, it was decided that a test would be required in order to have any confidence that it would work in practice (and not just be a waste of expensive fissile material).

After much preparation, the first nuclear test took place on 16 July 1945, near Alamogordo in New Mexico, under the supervision of Grove’s deputy, Brigadier General Thomas F. Farrell.

After the MAUD Committee’s report, the British and Americans exchanged nuclear information, but initially did not pool their efforts. A separate British 'Tube Alloys' project was started but did not have the resources of the 'Manhattan' project. Consequently the bargaining position of the British was worsened, and the British motives were mistrusted by the Americans. Collaboration therefore lessened markedly until the Quebec Agreement of August 1943, when a large team of British and Canadian scientists joined the 'Manhattan' Project.

Axis efforts on the bomb has always been an issue of contention, while it is believed that token efforts in Germany, headed by Werner Heisenberg, and in Japan were also undertaken during the war, but neither Axis effort made significant progress. It was initially feared that Germany was very close to developing a 'Nazi bomb'. Many captured Nazi scientists in fact expressed surprise to their Allied captors after they had learned of the detonation of the bombs over Japan, and they had been sure that the Allied effort was merely propaganda.

Together with the cryptanalysis efforts centred at Bletchley Park in the UK and at Arlington Hall in the USA, the development of radar and computers in the UK and later in the USA, and the turbojet engine in the UK and Germany, the 'Manhattan' Project represents one of few massive, secret and outstandingly successful technological efforts of World War II.