1958 US–UK Mutual Defence Agreement 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. The first British atomic bomb was successfully tested in Operation Hurricane on 3 October 1952, but although it was more advanced than the American bombs of 1946, Britain was still several years behind in nuclear weapons technology. The United States conducted Ivy Mike, the first nuclear test of a true thermonuclear device (also known as a hydrogen
Operation Upshot–Knothole Operation Upshot–Knothole Operation Upshot–Knothole was a series of eleven nuclear test shots conducted in 1953 at the Nevada Test Site. It followed "Operation Ivy" and preceded "Operation Castle". Over 21,000 soldiers took part in the ground exercise Desert Rock V in conjunction with the "Grable" shot. "Grable" was a 280mm shell fired from the "Atomic Cannon" and was viewed by a number of high-ranking military officials. The test series was notable as containing the first time an atomic artillery shell was fired (shot "Grable"), the first two shots (both fizzles) by University of California Radiation Laboratory—Livermore (now Lawrence Livermore National