The US–UK Mutual Defense Agreement, or the 1958 UK–US Mutual Defence Agreement, is a bilateral treaty between the United States and the United Kingdom on nuclear weapons co-operation. The treaty's full name is Agreement between the Government of the United States of America and the Government of the United Kingdom of Great Britain and Northern Ireland for Cooperation on the uses of Atomic Energy for Mutual Defense Purposes. It allows the US and the UK to exchange nuclear materials, technology and information. The US has nuclear co-operation agreements with other countries, including France and other NATO countries, but this agreement is by far the most comprehensive. Because of the agreement's strategic value to the United Kingdom, Harold Macmillan (the prime minister who presided over the United Kingdom's entry into the agreement) called it "the Great Prize".
The treaty was signed on 3 July 1958 after the Soviet Union had shocked the American public with the launch of Sputnik on 4 October 1957, and the British hydrogen bomb programme had successfully tested a thermonuclear device in the Operation Grapple test on 8 November. The special relationship proved mutually beneficial, both militarily and economically. The United Kingdom soon became dependent on the United States for its nuclear weapons since it agreed to limit their nuclear program with the agreement of shared technology. The treaty allowed American nuclear weapons to be supplied to the United Kingdom through Project E for use by the Royal Air Force and British Army of the Rhine until the early 1990s when the UK became fully independent in designing and manufacturing its own warheads.
The treaty provided for the sale to the UK of one complete nuclear submarine propulsion plant, as well as ten years' supply of enriched uranium to fuel it. Other nuclear material was also acquired from the US under the treaty. Some 5.4 tonnes of UK-produced plutonium was sent to the US in return for of tritium and 7.5 tonnes of highly enriched uranium (HEU) between 1960 and 1979, but much of the HEU was used not for weapons but as fuel for the growing fleet of British nuclear submarines. The treaty paved the way for the Polaris Sales Agreement, and the Royal Navy ultimately acquired entire weapons systems, with the UK Polaris programme and Trident nuclear programme using American missiles with British nuclear warheads.
The treaty has been amended and renewed ten times. On 14 November 2024, it was extended indefinitely.
Background Quebec Agreement
During the early part of the Second World War, the United Kingdom had a nuclear weapons project, codenamed Tube Alloys. At the Quadrant Conference in August 1943, the prime minister of the United Kingdom, Winston Churchill, and the president of the United States, Franklin Roosevelt, signed the Quebec Agreement, which merged Tube Alloys with the American Manhattan Project to create a combined British, American and Canadian project. The Quebec Agreement established the Combined Policy Committee and the Combined Development Trust to co-ordinate their efforts. Many of United Kingdom top scientists participated in the Manhattan Project.
The September 1944 Hyde Park Aide-Mémoire extended both commercial and military co-operation into the post-war period, but Roosevelt died on 12 April 1945, and it was not binding on subsequent administrations. In fact, it was physically lost. When Field Marshal Sir Henry Maitland Wilson raised the matter in a Combined Policy Committee meeting in June 1945, the American copy could not be found. The Quebec Agreement specified that nuclear weapons would not be used against another country without mutual consent. On 4 July, Wilson gave the British agreement for the use of nuclear weapons against Japan. On 8 August, the prime minister, Clement Attlee, sent a message to President Harry Truman that referred to themselves as "heads of the Governments which have control of this great force".
Truman administration The British government had trusted that the US would continue to share nuclear technology, which it considered to be a joint discovery. On 9 November 1945, Attlee and the prime minister of Canada, Mackenzie King, went to Washington, DC, to confer with Truman about future co-operation in nuclear weapons and nuclear power. A Memorandum of Intention was signed on 16 November 1945 that made Canada a full partner and replaced the Quebec Agreement's requirement for "mutual consent" before using nuclear weapons to one for "prior consultation". There was to be "full and effective co-operation in the field of atomic energy", but British hopes were soon disappointed since it was only "in the field of basic scientific research".
Technical co-operation was ended by the United States Atomic Energy Act of 1946 (McMahon Act), which forbade passing "restricted data" to American allies under pain of death. That partly resulted from the arrest for espionage of the British physicist Alan Nunn May in February 1946 while the legislation was being debated. Fearing a resurgence of American isolationism and the United Kingdom losing its great power status, the British government restarted its own development effort, now codenamed High Explosive Research.
By the end of 1947, of uranium ore from the Belgian Congo was stockpiled for the Combined Development Trust at Springfields, near Preston in Lancashire, as part of a wartime sharing agreement, along with for British use. To gain access to the stockpile for their own nuclear weapons project, the Americans opened negotiations that resulted in the Modus Vivendi, an agreement that was signed on 7 January 1948 and officially terminated all previous agreements, including the Quebec Agreement. It removed the British right of consultation on the use of nuclear weapons; allowed for limited sharing of technical information between the United States, United Kingdom and Canada and continued the Combined Policy Committee and the Combined Development Trust although the latter was renamed the Combined Development Agency.
In 1949, the Americans offered to make atomic bombs in the US available for United Kingdom to use if the British agreed to curtail their atomic bomb programme. That would have given United Kingdom nuclear weapons much sooner than its own target date of late 1952. Only the bomb components required by war plans would be stored in the UK, the rest would be kept in the US and Canada. The offer was rejected by the British on the grounds that it was not "compatible with our status as a first-class power to depend on others for weapons of this supreme importance".
As a counter-offer, the British proposed limiting the British programme in return for American bombs. The opposition of key American officials, including Lewis Strauss from the United States Atomic Energy Commission (AEC), and Senators Bourke B. Hickenlooper and Arthur Vandenberg of the Joint Committee on Atomic Energy (JCAE), coupled with security concerns aroused by the 2 February 1950 arrest of the British physicist Klaus Fuchs as an atomic spy, caused the proposal to be dropped. The June 1951 defection of Donald Maclean, who had served as a British member of the Combined Policy Committee from January 1947 to August 1948, reinforced the Americans' distrust of British security arrangements.
Eisenhower administration The first British atomic bomb was successfully tested in Western Australia in Operation Hurricane on 3 October 1952, but although it was more advanced than the American bombs of 1946, United Kingdom was still several years behind in nuclear weapons technology. On 1 November, the United States conducted Ivy Mike, the first nuclear test of a true thermonuclear device (also known as a hydrogen bomb). The JCAE saw little benefit for the US from sharing technology with the United Kingdom. The Soviet Union responded to Ivy Mike with the test of Joe 4, a boosted fission weapon, on 12 August 1953. That prompted President Dwight Eisenhower, who was inaugurated in January 1953, to inform the US Congress that the McMahon Act, which he considered a "terrible piece of legislation" and "one of the most deplorable incidents in American history of which he personally felt ashamed", was obsolete.
At the three-power Bermuda Conference in December 1953, Eisenhower and Churchill, who had become prime minister again on 25 October 1951, discussed the possibility of the United States giving the United Kingdom access to American nuclear weapons in wartime, which came to be called Project E. There were technical and legal issues that had to be overcome before American bombs could be carried in British aircraft. The Americans would have to disclose their weights and dimensions, and their delivery would require data concerning their ballistics. Further down the track would also be issues of custody, security and targeting. The release of such information was restricted by the McMahon Act.
It was amended on 30 August 1954 by the Atomic Energy Act of 1954, which allowed for greater exchange of information with foreign nations and paved the way for the Agreement for Co-operation Regarding Atomic Information for Mutual Defence Purposes, which was signed on 15 June 1955. On 13 June 1956, another agreement was concluded for the transfer of nuclear submarine propulsion technology to the United Kingdom, which saved the British government millions of pounds in research and development costs. It precipitated a row with the JCAE over whether that was permitted under the Atomic Energy Act of 1954 and whether United Kingdom met the security standards set by the 1955 agreement. With the 1956 presidential election approaching, Eisenhower was forced to rescind the offer.
The October 1956 Suez Crisis brought relations between the United Kingdom and the United States to a low ebb. Eisenhower met with the new British prime minister, Harold Macmillan, in Bermuda in March 1957 and raised the possibility of basing US intermediate range ballistic missile (IRBM) systems in the UK. This came to be called Project Emily. There were also discussions on exchanging nuclear submarine propulsion technology for information on the British Calder Hall nuclear power plant, allowing the United Kingdom Atomic Energy Authority (UKAEA) to purchase uranium ore from Canada and co-ordinating the war plans of RAF Bomber Command with those of the Strategic Air Command.
Although the IRBM negotiations pre-dated the Suez Crisis, the British government touted the IRBM deal as a demonstration that the rift had been healed. The British hydrogen bomb programme attempted to detonate a thermonuclear device in the Operation Grapple test series at Christmas Island in the Pacific. The test series was facilitated by the US, which also claimed the island. Although the first tests were unsuccessful, the Grapple X test on 8 November achieved the desired result.
Negotiations Sputnik crisis
The successful development of British thermonuclear weapons came at an opportune moment to renew negotiations with the Americans. The Soviet Union's launch of Sputnik 1, the world's first artificial satellite, on 4 October 1957, came as a tremendous shock to the American public, which had trusted that American technological superiority ensured their invulnerability. Suddenly, there was now incontrovertible proof that in at least some areas, the Soviet Union was actually ahead. In the widespread calls for action in response to the Sputnik crisis, officials in the United States and United Kingdom seized an opportunity to mend their relationship. At the suggestion of Harold Caccia, the British Ambassador to the United States, Macmillan wrote to Eisenhower on 10 October to urge for both countries pool their resources, as Macmillan put it, to meet the Soviet challenge on every front, "military, economic and political".