Manhattan Project
The Manhattan Project was the project to develop the first nuclear weapon (atomic bomb) during World War II by the United States, and the United Kingdom. Formally designated as the Manhattan Engineer District (MED), it refers specifically to the period of the project from 19411946 under the control of the U.S. Army Corps of Engineers, under the administration of General Leslie R. Groves. The scientific research was directed by American physicist J. Robert Oppenheimer. 
The project's roots lay in scientists' fears since the 1930s that Nazi Germany was also investigating nuclear weapons of its own. Born out of a small research program in 1939, the Manhattan Project eventually employed more than 130,000 people and cost nearly $2 billion USD ($24 billion in 2008 dollars based on CPI). It resulted in the creation of multiple production and research sites that operated in secret.
The three primary research and production sites of the project were the plutonium-production facility at what is now the Hanford Site, the uranium-enrichment facilities at Oak Ridge, Tennessee, and the weapons research and design laboratory, now known as Los Alamos National Laboratory. Project research took place at over thirty sites across the United States, and the United Kingdom. The MED maintained control over U.S. weapons production until the formation of the Atomic Energy Commission in January 1947.

# Origin of name
The project was coordinated from the U.S. Army Corps of Engineers North Atlantic Division headquarters on the 18th floor of 270 Broadway near New York City Hall in Manhattan.
The initial proposed name was "Laboratory for the Development of Substitute Materials."  Fearing the name would draw undue attention, General Leslie Groves changed it to the (secret) name "Manhattan Engineer District", for a non-existent administrative division of the US Army Corps of Engineers. In daily parlance, the nickname became the Manhattan Project.  The Corps Manhattan district, unlike other regional Corps offices, was not to have territorial limits.
Much of the early research was done at Columbia University in Pupin Hall and at one time employed 700 people on the project. 
Coordination for the project moved to Oak Ridge in 1943, but the name Manhattan Engineer District was not changed.

# Discovery of nuclear fission
The first decades of the twentieth century led to radical changes in the understanding of the physics of the atom, including the discovery of the nucleus, the idea of radiation, and the fact that the splitting of atomic nuclei could lead to massive release of energy (nuclear fission).
By 1932, the atom was thought to consist of a small, dense nucleus containing most of the atom's mass in the form of protons and neutrons and was surrounded by a shell of electrons. Study on the phenomenon of radioactivity began with the discovery of uranium ores by Henri Becquerel in 1896 and was followed by the work of Pierre and Marie Curie on radium. Their research seemed to promise that atoms, previously thought to be ultimately stable and indivisible, actually had the potential of containing and releasing immense amounts of energy. In 1919 Ernest Rutherford achieved the first artificial nuclear disintegrations by bombarding nitrogen with alpha particles emitted from a radioactive source, thus becoming the first person in history to intentionally "split the atom". It had become clear from the Curies' work that there was a tremendous amount of energy locked up in radioactive decayfar more than chemistry could account for. But even in the early 1930s such illustrious physicists as Ernest Rutherford and Albert Einstein could see no way of artificially releasing that energy any faster than nature naturally allowed it to leave. "Radium engines" in the 1930s were the stuff of science fiction, such as was being written at the time by Edgar Rice Burroughs. H. G. Wells included air-dropped "atomic bombs" in his 1914 novel The World Set Free. Though Wells' "atomic bombs" bore little resemblance to actual nuclear weapons (they were simply regular bombs that never stopped exploding), Leo Szilard later commented that this story influenced his later research into this subject.
Progress in controlling and understanding nuclear fission accelerated in the 1930s when further manipulation of the nuclei of atoms became possible.  In 1932, Sir John Cockcroft and Ernest Walton were first to "split the atom" (cause a nuclear reaction) by using artificially accelerated particles.  In 1934, Irene and Frederic Joliot-Curie discovered that artificial radioactivity could be induced in stable elements by bombarding them with alpha particles. The same year Enrico Fermi reported similar results when bombarding uranium with neutrons (discovered in 1932), but he did not immediately appreciate the consequences of his results.
In December 1938, the Germans Otto Hahn and Fritz Strassmann published experimental results about bombarding uranium with neutrons. They showed that it produced an isotope of barium. Shortly after, their Austrian co-worker Lise Meitner (a political refugee in Sweden at the time) and her nephew Otto Robert Frisch correctly interpreted the results as the splitting of the uranium nucleus after the absorption of a neutronnuclear fissionwhich released a large amount of energy and additional neutrons. A direct experimental evidence of the nuclear fission was performed by Frisch, following a fundamental idea suggested to him by George Placzek .
In 1933, Hungarian physicist Leo Szilard had proposed that if any neutron-driven process released more neutrons than those required to start it, an expanding nuclear chain reaction might result. Chain reactions were familiar as a phenomenon from chemistry (where they typically caused explosions and other run-away reactions), but Szilard was proposing them for a nuclear reaction, for the first time. However, Szilard had proposed to look for such reactions in the lighter atoms, and nothing of the sort was found. Upon experimentation shortly after the uranium fission discovery, Szilard found that the fission of uranium released two or more neutrons on average, and immediately realized that a nuclear chain reaction by this mechanism was possible in theory. Szilard kept this secret at first because he feared its use as a weapon by fascist governments. He convinced others to do so, but identical results were soon published by the Joliot Curie group, to his great dismay.
That such mechanisms might have implications for civilian power or military weapons was perceived by numerous scientists in many countries, around the same time. While these developments in science were occurring, many political changes were happening in Europe. Adolf Hitler was appointed chancellor of Germany in January 1933. His anti-Semitic ideology caused all Jewish civil servants, including many physicists, to be fired from their posts. Consequently many European physicists who later made key discoveries went into exile in the United Kingdom and the United States. After Nazi Germany invaded Poland in 1939 and World War II began, many scientists in the United States and the United Kingdom became anxious about what Germany might do with nuclear technology.  Albert Einstein in particular wrote several letters to Franklin Roosevelt urging him to establish the nuclear capability before the Germans.  These letters, especially one called the Einstein-Szilard letter (written in August 1939, but not personally received by Roosevelt until October 1939), are considered to be influential in the acceleration of the projectTemplate:Who.

# Acceleration of the Project
Having begun to wrest control of the uranium research from the National Bureau of Standards, the project leaders began to accelerate the bomb project under the OSRD. Arthur Compton organized the University of Chicago Metallurgical Laboratory in early 1942 to study plutonium and fission piles (primitive nuclear reactors), and asked theoretical physicist Robert Oppenheimer of the University of California, Berkeley to take over research on fast neutron calculationskey to calculations about critical mass and weapon detonationfrom Gregory Breit. John Manley, a physicist at the Metallurgical Laboratory, was assigned to help Oppenheimer find answers by coordinating and contacting several experimental physics groups scattered across the country.
During the spring of 1942, Oppenheimer and Robert Serber of the University of Illinois worked on the problems of neutron diffusion (how neutrons moved in the chain reaction) and hydrodynamics (how the explosion produced by the chain reaction might behave). To review this work and the general theory of fission reactions, Oppenheimer convened a summer study at the University of California, Berkeley, in June 1942. Theorists Hans Bethe, John Van Vleck, Edward Teller, Felix Bloch, Emil Konopinski, Robert Serber, Stanley S. Frankel, and Eldred C. Nelson (the latter three all former students of Oppenheimer) quickly confirmed that a fission bomb was feasible. 
There were still many unknown factors in the development of a nuclear bomb, however, even though it was considered to be theoretically possible. The properties of pure uranium-235 were still relatively unknown, as were the properties of plutonium, a new element which had only been discovered in February 1941 by Glenn Seaborg and his team. Plutonium was the product of uranium-238 absorbing a neutron which had been emitted from a fissioning uranium-235 atom, and was thus able to be created in a nuclear reactor. But at this point no reactor had yet been built, so while plutonium was being pursued as an additional fissile substance, it was not yet to be relied upon. Only microgram quantities of plutonium existed at the time (produced from neutrons derived from reaction started in a cyclotron).
The scientists at the Berkeley conference determined that there were many possible ways of arranging the fissile material into a critical mass, the simplest being the shooting of a "cylindrical plug" into a sphere of "active material" with a "tamper"dense material which would focus neutrons inward and keep the reacting mass together to increase its efficiency (this model "avoids fancy shapes", Serber would later write). They also explored designs involving spheroids, a primitive form of "implosion" (suggested by Richard C. Tolman), and explored the speculative possibility of "autocatalytic methods" which would increase the efficiency of the bomb as it exploded.
Considering the idea of the fission bomb theoretically settled until more experimental data were available, the conference then turned in a different direction. Hungarian physicist Edward Teller pushed for discussion on an even more powerful bomb: the "Super", which would use the explosive force of a detonating fission bomb to ignite a fusion reaction in deuterium and tritium. This concept was based on studies of energy production in stars made by Hans Bethe before the war, and suggested as a possibility to Teller by Enrico Fermi not long before the conference. 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 could.  But Bethe was skeptical.  As Teller pushed hard for his "superbomb"now usually referred to as a "hydrogen bomb"proposing scheme after scheme, Bethe refuted each one. The fusion idea had to be put aside in order to concentrate on actually producing fission bombs.
Teller also raised the speculative possibility that an atomic bomb might "ignite" the atmosphere, because of a hypothetical fusion reaction of nitrogen nuclei. Bethe calculated, according to Serber, that it could not happen.  In his book The Road from Los Alamos, Bethe says a refutation was written by Konopinski, C. Marvin, and Teller as report LA-602, showing that ignition of the atmosphere was impossible, not just unlikely. In Serber's account, Oppenheimer mentioned it 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 being "never laid to rest".
The conferences in the summer of 1942 provided the detailed theoretical basis for the design of the atomic bomb, and convinced Oppenheimer of the benefits of having a single centralized laboratory to manage the research for the bomb project, rather than having specialists spread out at different sites across the United States.

# Project sites
Though it involved over thirty different research and production sites, the Manhattan Project was largely carried out at three secret scientific cities that were established by power of eminent domain: Los Alamos, New Mexico; Oak Ridge, Tennessee; and Richland, Washington. The Tennessee site was chosen for the vast quantities of cheap hydroelectric power already available there (due to the Tennessee Valley Authority) necessary to produce uranium-235 in giant ion separation magnets. The Hanford Site near Richland, Washington, was chosen for its location near a river that could supply water to cool the reactors which would produce the plutonium. All the sites were suitably far from coastlines and therefore less vulnerable to possible enemy attack from Germany or Japan.
The Los Alamos National Laboratory was built on a mesa that previously hosted the Los Alamos Ranch School, a private school for teenage boys. The site was chosen primarily for its remoteness. Oppenheimer had known of it from his horse-riding near his ranch in New Mexico, and he showed it as a possible site to the government representatives, who promptly bought it for $440,000. In addition to being the main "think-tank", Los Alamos was responsible for final assembly of the bombs, mainly from materials and components produced by other sites. Manufacturing at Los Alamos included casings, explosive lenses, and fabrication of fissile materials into bomb cores.
Oak Ridge facilities covered more than 60,000 acres (243 km2) of several former farm communities in the Tennessee Valley area. Some Tennessee families were given two weeks' notice to vacate family farms that had been their home for generations. So secret was the site during WW2 that the state governor was unaware that Oak Ridge (which was to become the fifth largest city in the state) was being built. At one point Oak Ridge plants were consuming 1/6th of the electrical power produced in the U.S., more than New York City. Oak Ridge mainly produced uranium-235.
The Hanford Site, which grew to almost 1,000 square miles (2,600 km2), took over irrigated farm land, fruit orchards, a railroad, and two farming communities, Hanford and White Bluffs, in a sparsely populated area adjacent to the Columbia River. Hanford hosted nuclear reactors cooled by the river and was the plutonium production center.
The existence of these sites and the secret cities of Los Alamos, Oak Ridge, and Richland were not made public until the announcement of the Hiroshima explosion, and the sites remained secret until after the end of WWII.
The project originally was headquartered at 270 Broadway in Manhattan. Other offices were scattered throughout the city.  The Broadway headquarters lasted little more than a year before it was moved in 1943, although many of the other offices in Manhattan remained.
Major Manhattan Project sites and subdivisions included:
- Site W (Hanford, Washington): a plutonium production facility (now Hanford Site)
- Site X (Oak Ridge, Tennessee): enriched uranium production and plutonium production research (now Oak Ridge National Laboratory) Site X also included:
X-10 Graphite Reactor: graphite reactor research pilot plant (on the site of what is now Oak Ridge National Laboratory)
Y-12: electromagnetic separation uranium enrichment plant
K-25: gaseous diffusion uranium enrichment plant
S-50: thermal diffusion uranium enrichment plant
- X-10 Graphite Reactor: graphite reactor research pilot plant (on the site of what is now Oak Ridge National Laboratory)
- Y-12: electromagnetic separation uranium enrichment plant
- K-25: gaseous diffusion uranium enrichment plant
- S-50: thermal diffusion uranium enrichment plant
- Site Y (Los Alamos, New Mexico): a bomb research laboratory (now Los Alamos National Laboratory)
- Metallurgical Laboratory (Chicago, Illinois): reactor development (now Argonne National Laboratory)
- Project Alberta (Wendover, Utah and Tinian): preparations for the combat delivery of the bombs
- Project Ames (Ames, Iowa): production of raw uranium metal (now Ames Laboratory)
- Dayton Project (Dayton, Ohio): research and development of polonium refinement and industrial production of polonium for atomic bomb triggers
- Project Camel (Inyokern, California): high explosives research and non-nuclear engineering for the Fat Man bomb
- Project Trinity (Alamogordo, New Mexico): preparations for the testing of the first atomic bomb
- Radiation Laboratory (Berkeley, California): electromagnetic separation enrichment research (now Lawrence Berkeley National Laboratory)
- Project '9' (Trail, British Columbia): heavy water (deuterium) production.

# Need for coordination
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 1942.
The need for better coordination was clear. By September 1942, the difficulties in conducting studies on nuclear weapons at universities scattered throughout the country indicated the need for a laboratory dedicated solely to that purpose. A greater need was the construction of industrial plants to produce uranium-235 and plutoniumthe fissionable materials to be used in the weapons.
Vannevar Bush, the head of the civilian Office of Scientific Research and Development (OSRD), asked President Roosevelt to assign the operations connected with the growing nuclear weapons project to the military. Roosevelt chose the Army to work with the OSRD in building production plants. The Army Corps of Engineers selected Col. James Marshall to oversee the construction of factories to separate uranium isotopes and manufacture plutonium for the bomb.
Marshall and his deputy, Col. Kenneth Nichols, struggled to understand the proposed processes and the scientists with whom they had to work. Thrust 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, Tennessee, would be suitable for the first production plant, they did not know how large the site needed to be and delayed its acquisition.
Because of its experimental nature, the nuclear weapons work could not compete with the Army's more urgent tasks for priority. The scientists' work and production plant construction often were delayed by Marshall's inability to obtain critical materials, such as steel, needed in other military projects.
Even selecting a name for the project was difficult. The title chosen by Gen. Brehon B. Somervell, "Development of Substitute Materials," was objectionable because it seemed to reveal too much.

# Manhattan Engineer District
Vannevar Bush became dissatisfied with Col. James Marshall's failure to get the project moving forward expeditiously and made this known to Secretary of War Stimson and Army Chief of Staff George Marshall. Marshall then directed General Somervell to replace Col. Marshall with a more energetic officer as director. In the summer of 1942, Col. Leslie Groves was deputy to the chief of construction for the Army Corps of Engineers and had overseen the very rapid construction of the Pentagon, the world's largest office building. He was widely respected as an intelligent, hard driving, though brusque officer who got things done in a hurry. Hoping for an overseas command, Groves vigorously objected when Somervell appointed him to the weapons project. His objections were overruled, and Groves resigned himself to leading a project he thought had little chance of success. Groves appointed Oppenheimer as the project's scientific director, to the surprise of many. (Oppenheimer's radical political views were thought to pose security problems). However, Groves was convinced Oppenheimer was a genius who could talk about and understand nearly anything, and he was convinced such a man was needed for a project such as the one being proposed.
Groves renamed the project The Manhattan Engineer District. The name evolved from the Corps of Engineers practice of naming districts after its headquarters' city (Marshall's headquarters were in New York City). At that time, Groves was promoted to brigadier general, giving him the rank necessary to deal with 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 was soon to become all too familiar to the atomic scientists.
The first major scientific hurdle of the project was solved on December 2, 1942, beneath the bleachers of Stagg Field at the University of Chicago, where a team led by Enrico Fermi, for whom Fermilab is named, initiated the first artificial  self sustaining nuclear chain reaction in an experimental nuclear reactor named Chicago Pile-1. A coded phone call from Compton saying, "The Italian navigator  has landed in the new world, the natives are friendly" to Conant in Washington, D.C., brought news of the experiment's success.

# Uranium bomb
The Hiroshima bomb, Little Boy, was made from uranium-235, a rare isotope of uranium that has to be physically separated from the more plentiful uranium-238 isotope, which is not suitable for use in an explosive device. Since U-235 is only 0.7% of raw uranium and is chemically identical to the 99.3% of U-238, various physical methods were considered for separation.
One method of separating uranium 235 from raw uranium ore was devised by Franz Simon and Nicholas Kurti, two Jewish emigres, at Oxford University.  Their method using gaseous diffusion was scaled up in large separation plants at Oak Ridge Laboratories and used uranium hexafluoride (UF6) gas as the process fluid. This method eventually produced most of the U-235, although it was also important for producing partly enriched material to feed the calutrons (see below), which also produced significant U-235.
Another methodelectromagnetic isotope separationwas developed by Ernest Lawrence at the University of California Radiation Laboratory at the University of California, Berkeley.  This method employed devices known as calutrons, which were effectively mass spectrometers. A total of 70,000,000 pounds of silver from the U.S. Treasury reserves was used for coils. (Copper was originally intended for the coils, but there was an insufficient amount available due to war shortages. The project engineers were forced to borrow silver from the Treasury, which was returned after the project ended.) Initially the method seemed promising for large scale production but was expensive and produced insufficient material and was later abandoned after the war.
Other techniques were also tried, such as thermal diffusion. Most of this separation work was performed at Oak Ridge.
The uranium bomb was a gun-type fission weapon. One mass of U-235, the "bullet,"  is fired down a more or less conventional gun barrel into another mass of U-235, rapidly creating the critical mass of U-235, resulting in an explosion. The method was so certain to work that no test was carried out before the bomb was dropped over Hiroshima. Also, the bomb dropped used all the existing extremely highly purified U-235 (and even most of the highly purified material) so there was no U-235 available for such a test anyway. The bomb's design was known to be inefficient and prone to accidental discharge. It has been estimated that only about 15% of the fissile material went critical.

# Plutonium bomb
The bombs used in the first test at Trinity Site on July 16 1945, in New Mexico (the gadget of the Trinity test), and in the Nagasaki bomb, Fat Man, were made primarily of plutonium-239, a synthetic element.
Although uranium-238 is useless as fissile isotope for an atomic bomb, U-238 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 plutonium-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 the Hanford Site.
From 19431944, development efforts were directed to a gun-type fission weapon with plutonium, called "Thin Man". Once this was achieved, the uranium version "Little Boy" would require a relatively simple adaptation, it was thought.
Initial tests of the properties of plutonium were done using cyclotron-generated plutonium-239, very pure but in very small amounts. On April 5 1944, Emilio Segre at Los Alamos received the first sample of Hanford-produced plutonium. Within ten days, he discovered a fatal flaw: reactor-bred plutonium was far less isotopically pure than cyclotron-produced plutonium, and as a result had a much higher spontaneous fission rate than uranium-235. The unwanted isotope responsible for this high fission rate was plutonium-240, formed from plutonium-239 by capture of an additional neutron. Unlike the cyclotron, the plutonium breeding reactors had a much higher neutron flux and thus yielded an increased proportion of plutonium-240. Plutonium-240 was even harder to separate from plutonium-239 than U-235 was to separate from U-238, so there was no question of doing so. The contaminating Pu-240 had to stay in the plutonium metal used in the bomb, where its spontaneous fissions were a source of unwanted neutrons. The implications of this made a "gun" detonation mechanism unsuitable. Because of the relatively slow speed of the gun device, "early" neutrons from spontaneously fissioning Pu-240 would start the reaction before the device was fully assembled by the gun process, and as a result, a plutonium bomb would "fizzle" (that is, heat up and blow itself apart) before it could be turned into a shape suitable for an efficient chain reaction which would split a substantial amount of the plutonium. Even a 1% fission of the material would result in a workable bomb, almost a thousand times more powerful than conventional bombs for the weight; but a fizzle promised far less even than this.
In July 1944, the 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, which had few complications. Most efforts were then 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"a sub-critical sphere of fissile material could, using chemical explosives, be forced to collapse in on itself, creating a very dense critical mass, which because of the very short distances the metal needed to travel to make it, would come into existence for a far shorter time than it would take to assemble a mass from a bullet. Initially, implosion had been entertained as a possible, though unlikely method. However, after it was discovered that it was the only possible solution for using reactor-bred plutonium, and that uranium-235 production could not be substantially increased, the implosion project received the highest priority, as the only solution to scaling up fissionable material production to the level needed for multiple bombs. By the end of July 1944, the entire project had been reorganized around solving the implosion problem. It eventually involved using shaped charges with many explosive lenses to produce the perfectly spherical explosive wave needed to properly compress the plutonium sphere.
Because of the complexity of an implosion-style weapon, it was decided that, despite the waste of fissile material, an initial test would be required. The first nuclear test took place on July 16 1945, near Alamogordo, New Mexico, under the supervision of Groves's deputy Brig. Gen. Thomas Farrell. This test was dubbed by Oppenheimer "Trinity".

# Similar efforts
A similar effort was undertaken in the USSR in September 1941 headed by Igor Kurchatov (with some of Kurchatov's World War II knowledge coming secondhand from Manhattan Project countries, thanks to spies, including at least two on the scientific team at Los Alamos, Klaus Fuchs and Theodore Hall, unknown to each other).
After the MAUD Committee's report, the British and Americans exchanged nuclear information but initially did not pool their efforts. A British project, code-named Tube Alloys, was started but did not have American resources.  Consequently the British bargaining position worsened, and their motives were mistrusted by the Americans.  Collaboration therefore lessened markedly until the Quebec Agreement of August 1943, when a large team of British, Canadian and Australian scientists joined the Manhattan Project.
The question of Axis efforts on the bomb has been a contentious issue for historians. It is believed that efforts undertaken in Germany, headed by Werner Heisenberg, and in Japan, were also undertaken during the war with little progress. It was initially feared that Hitler was very close to developing his own bomb. Many German scientists in fact expressed surprise to their Allied captors when the bombs were detonated in Japan. They were convinced that talk of atomic weapons was merely propaganda. However, Werner Heisenberg (by then imprisoned in England at Farm Hall with several other nuclear project physicists) almost immediately figured out what the Allies had done, explaining it to his fellow scientists (and hidden microphones) within days. The Nazi reactor effort had been severely handicapped by Heisenberg's belief that heavy water was necessary as a neutron moderator (slowing preparation material) for such a device. The Germans were short of heavy water throughout the war because of Allied efforts to prevent Germany from obtaining it, and the Germans never did stumble on the secret of purified graphite for making nuclear reactors from natural uranium.
Bohr, Heisenberg and Fermi were all colleagues who were key figures in developing the quantum theory together with Wolfgang Pauli, prior to the war. They had known each other well in Europe and were friends. Niels Bohr and Heisenberg even discussed the possibility of the atomic bomb prior to and during the war, before the United States became involved. Bohr recalled that Heisenberg was unaware that the supercritical mass could be achieved with U-235, and both men gave differing accounts of their conversations at this sensitive time. Bohr at the time did not trust Heisenberg, and never quite forgave him for his decision not to flee Germany before the war when given the chance. Heisenberg, for his part, seems to have thought he was proposing to Bohr a mutual agreement between the two sides not to pursue nuclear technology for destructive purposes. If so, Heisenberg's message did not get through.  Heisenberg, to the end of his life, maintained that the partly-built German heavy-water nuclear reactor found after the war's end in his lab was for research purposes only, and a full bomb project had not been contemplated (there is no evidence to contradict this, but by this time late in the war, Germany was far from having the resources for a Hanford-style plutonium bomb, even if its scientists had decided to pursue one and had known how to do it).
Together with the cryptographic efforts centered at Bletchley Park and also at Arlington Hall, the development of radar and computers in the UK and later in the US, and the jet engine in the UK and Germany, the Manhattan Project represents one of the few massive, secret and outstandingly successful technological efforts spawned by the conflict of World War II.