1958 US–UK Mutual Defence Agreement  The 1958 US–UK Mutual Defense Agreement, or 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 United States and the UK to exchange nuclear materials, technology and information. While the US has nuclear co-operation agreements with other countries, including France and other NATO countries, this agreement is by far the most comprehensive. Because of the agreement's strategic value to Britain, 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 shocked the American public with the launch of Sputnik on 4 October 1957, and the British hydrogen bomb programme successfully tested a thermonuclear device in the Operation Grapple test on 8 November. The Anglo-American Special Relationship proved mutually beneficial, although it was never one of equals after the World Wars; the United States was (and is) far larger than Britain both militarily and economically. Britain soon became dependent on the United States for its nuclear weapons, as it lacked the resources to produce a range of designs. The treaty allowed American nuclear weapons to be supplied to Britain through Project E, for use by the Royal Air Force and British Army of the Rhine. The treaty provided for the sale to the UK of one complete nuclear submarine propulsion plant, plus ten years' supply of enriched uranium to fuel it. Other nuclear material was also acquired from the United States 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, although much of the HEU was used not for weapons, but as fuel for the growing fleet of UK 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 nine times. The most recent renewal extended it to 31 December 2024. During the early part of the Second World War, Britain 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.