November 1945 agreement) on 12 May 1947, resulting in intense pressure on Truman to drop the provision. On 7 January 1948, Bush, James Fisk, Cockcroft and Mackenzie concluded an agreement known as the modus vivendi, that allowed for limited sharing of technical information between the United States, Britain and Canada, which officially repealed the Quebec Agreement. Like the Quebec Agreement it replaced, the modus vivendi was classified "Top Secret". As the Cold War set in, enthusiasm in the United States for an alliance with Britain cooled as well. A September 1949 poll found that 72 per cent of Americans agreed that the United States should not "share our atomic energy secrets with England". The reputation of the British was further tarnished by the 1950 revelation that Fuchs was a Soviet atomic spy. British wartime participation in the Manhattan Project provided a substantial body of expertise that was crucial to the success of High Explosive Research, the United Kingdom's post-war nuclear weapons programme, although it was not without important gaps, such as in the field of plutonium metallurgy. The development of the independent British nuclear deterrent led to the McMahon Act being amended in 1958, and to a resumption of the nuclear Special Relationship between America and Britain under the 1958 US–UK Mutual Defence Agreement. Footnotes Notes References External links Quebec Agreement on Yale Law School site Hyde Park Aide-Mémoire on US State Department site United Kingdom–United States treaties World War II treaties Treaties concluded in 1943 Secret treaties Canada–United Kingdom relations Canada–United States relations Nuclear weapons policy Nuclear history of the United Kingdom History of the Manhattan Project United Kingdom in World War II August 1943 events 1943 in Canada 1943 in the United Kingdom 1943 in the United States 20th century in Quebec City 1940s in Quebec