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.