Britain initiated the world's first research project to design an atomic bomb in 1941. Building on this work, Britain prompted the United States to recognise how important this type of research was, helped the U.S. to start the Manhattan Project in 1942, and supplied crucial expertise and materials that contributed to the project's successful completion in time to influence the end of the Second World War.

Following the discovery of nuclear fission in uranium, scientists Rudolf Peierls and Otto Frisch at the University of Birmingham calculated, in March 1940, that the critical mass of a metallic sphere of pure uranium-235 was as little as , and would explode with the power of thousands of tons of dynamite. The Frisch–Peierls memorandum prompted Britain to create an atomic bomb project, known as Tube Alloys. Mark Oliphant, an Australian physicist working in Britain, was instrumental in making the results of the British MAUD Report known in the United States in 1941 by a visit in person. Initially the British project was larger and more advanced, but after the United States entered the war, the American project soon outstripped and dwarfed its British counterpart. The British government then decided to shelve its own nuclear ambitions, and participate in the American project.

In August 1943, the prime minister of the United Kingdom, Winston Churchill, and the president of the United States, Franklin D. Roosevelt, signed the Quebec Agreement, which provided for cooperation between the two countries. The Quebec Agreement established the Combined Policy Committee and the Combined Development Trust to coordinate the efforts of the United States, the United Kingdom and Canada. The subsequent Hyde Park Agreement in September 1944 extended this cooperation to the postwar period. A British Mission led by Wallace Akers assisted in the development of gaseous diffusion technology in New York. Britain also produced the powdered nickel required by the gaseous diffusion process. Another mission, led by Oliphant who acted as deputy director at the Berkeley Radiation Laboratory, assisted with the electromagnetic separation process. As head of the British Mission to the Los Alamos Laboratory, James Chadwick led a multinational team of distinguished scientists that included Sir Geoffrey Taylor, James Tuck, Niels Bohr, Peierls, Frisch, and Klaus Fuchs, who was later revealed to be a Soviet atomic spy. Four members of the British Mission became group leaders at Los Alamos. William Penney observed the bombing of Nagasaki and participated in the Operation Crossroads nuclear tests in 1946.

Cooperation ended with the Atomic Energy Act of 1946, known as the McMahon Act, and Ernest Titterton, the last British government employee, left Los Alamos on 12 April 1947. Britain then proceeded with High Explosive Research, its own nuclear weapons programme, and became the third country to test an independently developed nuclear weapon in October 1952.

Background

The 1938 discovery of nuclear fission in uranium by Otto Robert Frisch, Fritz Strassmann, Lise Meitner and Otto Hahn, raised the possibility that an extremely powerful atomic bomb could be created. Refugees from Nazi Germany and other fascist countries were particularly alarmed by the notion of a German nuclear weapon project. In the United States, three of them, Leo Szilard, Eugene Wigner and Albert Einstein, were moved to write the Einstein–Szilárd letter to the President of the United States, Franklin D. Roosevelt, warning of the danger. This led to the President creating the Advisory Committee on Uranium. In Britain, Nobel Prize in Physics laureates George Paget Thomson and William Lawrence Bragg were sufficiently concerned to take up the matter. Their concerns reached the Secretary of the Committee of Imperial Defence, Major General Hastings Ismay, who consulted with Sir Henry Tizard. Like many scientists, Tizard was sceptical of the likelihood of an atomic bomb being developed, reckoning the odds against success at 100,000 to 1.

Even at such long odds, the danger was sufficiently great to be taken seriously. Thomson, at Imperial College London, and Mark Oliphant, an Australian physicist at the University of Birmingham, were tasked with carrying out a series of experiments on uranium. By February 1940, Thomson's team had failed to create a chain reaction in natural uranium, and he had decided that it was not worth pursuing. But at Birmingham, Oliphant's team had reached a strikingly different conclusion. Oliphant had delegated the task to two German refugee scientists, Rudolf Peierls and Otto Frisch, who could not work on the university's radar project because they were enemy aliens and therefore lacked the necessary security clearance. They calculated the critical mass of a metallic sphere of pure uranium-235, the only fissile isotope found in significant quantity in nature, and found that instead of tons, as everyone had assumed, as little as would suffice, which would explode with the power of thousands of tons of dynamite.

Oliphant took the Frisch–Peierls memorandum to Tizard, and the MAUD Committee was established to investigate further. It directed an intensive research effort, and in July 1941, produced two comprehensive reports that reached the conclusion that an atomic bomb was not only technically feasible, but could be produced before the war ended, perhaps in as little as two years. The Committee unanimously recommended pursuing the development of an atomic bomb as a matter of urgency, although it recognised that the resources required might be beyond those available to Britain. A new directorate known as Tube Alloys was created to coordinate this effort. Sir John Anderson, the Lord President of the Council, became the minister responsible, and Wallace Akers from Imperial Chemical Industries (ICI) was appointed the director of Tube Alloys.

Contribution Early Anglo-American cooperation In July 1940, Britain had offered to give the United States access to its scientific research, and the Tizard Mission's John Cockcroft briefed American scientists on British developments. He discovered that the American project was smaller than the British, and not as far advanced. As part of the scientific exchange, the Maud Committee's findings were conveyed to the United States. Oliphant, one of the Maud Committee's members, flew to the United States in late August 1941, and discovered that vital information had not reached key American physicists. He met the Uranium Committee, and visited Berkeley, California, where he spoke persuasively to Ernest O. Lawrence, who was sufficiently impressed to commence his own research into uranium at the Berkeley Radiation Laboratory. Lawrence in turn spoke to James B. Conant, Arthur H. Compton and George B. Pegram. Oliphant's mission was a success; key American physicists became aware of the potential power of an atomic bomb. Armed with British data, Vannevar Bush, the director of the Office of Scientific Research and Development (OSRD), briefed Roosevelt and Vice President Henry A. Wallace in a meeting at the White House on 9 October 1941.

The British and Americans exchanged nuclear information, but did not initially combine their efforts. British officials did not reply to an August 1941 offer by Bush and Conant to create a combined British and American project. In November 1941, Frederick L. Hovde, the head of the London liaison office of OSRD, raised the issue of cooperation and exchange of information with Anderson and Lord Cherwell, who demurred, ostensibly over concerns about American security. Ironically, it was the British project that had already been penetrated by atomic spies for the Soviet Union.

Yet the United Kingdom did not have the manpower or resources of the United States, and despite its early and promising start, Tube Alloys fell behind its American counterpart and was dwarfed by it. Britain was spending around £430,000 per year on research and development, and Metropolitan-Vickers was building gaseous diffusion units for uranium enrichment worth £150,000; but the Manhattan Project was spending £8,750,000 on research and development, and had let construction contracts worth £100,000,000 at the fixed wartime rate of four dollars to the pound. On 30 July 1942, Anderson advised the prime minister of the United Kingdom, Winston Churchill, that: "We must face the fact that ... [our] pioneering work ... is a dwindling asset and that, unless we capitalise it quickly, we shall be outstripped. We now have a real contribution to make to a 'merger.' Soon we shall have little or none."

By then, the positions of the two countries had reversed from what they were in 1941. The Americans had become suspicious that the British were seeking commercial advantages after the war, and Brigadier General Leslie R. Groves Jr., who took over command of the Manhattan Project on 23 September 1942, wanted to tighten security with a policy of strict compartmentalisation similar to the one that the British had imposed on radar. American officials decided that the United States no longer needed outside help. The Secretary of War, Henry L. Stimson, felt that since the United States was doing "ninety percent of the work" on the bomb, it would be "better for us to go along for the present without sharing anything more than we could help". In December 1942, Roosevelt agreed to restricting the flow of information to what Britain could use during the war, even if doing so slowed down the American project. In retaliation, the British stopped sending information and scientists to America, and the Americans then stopped all information sharing.

The British considered how they would produce a bomb without American help. A gaseous diffusion plant to produce of weapons-grade uranium per day was estimated to cost up to £3,000,000 in research and development, and anything up to £50,000,000 to build in wartime Britain. A nuclear reactor to produce of plutonium per day would have to be built in Canada. It would take up to five years to build and cost £5,000,000. The project would also require facilities for producing the required heavy water for the reactor costing between £5,000,000 and £10,000,000, and for producing uranium metal £1,500,000. The project would need overwhelming priority, as it was estimated to require 20,000 workers, many of them highly skilled, 500,000 tons of steel, and 500,000 kW of electricity. Disruption to other wartime projects would be inevitable, and it was unlikely to be ready in time to affect the outcome of the war in Europe. The unanimous response was that before embarking on this, another effort should be made to obtain American cooperation.

Cooperation resumes

By March 1943 Conant decided that British help would benefit some areas of the project. In particular, the Manhattan Project could benefit enough from assistance from James Chadwick, the discoverer of the neutron, and several other British scientists to warrant the risk of revealing weapon design secrets. Bush, Conant and Groves wanted Chadwick and Peierls to discuss bomb design with Robert Oppenheimer, and Kellogg still wanted British comments on the design of the gaseous diffusion plant.

Churchill took up the matter with Roosevelt at the Washington Conference on 25 May 1943, and Churchill thought that Roosevelt gave the reassurances he sought; but there was no follow-up. Bush, Stimson and William Bundy met Churchill, Cherwell and Anderson at 10 Downing Street in London. None of them was aware that Roosevelt had already made his decision, writing to Bush on 20 July 1943 with instructions to "renew, in an inclusive manner, the full exchange with the British Government regarding Tube Alloys."