During the Second World War, Britain and America cooperated closely in the development of the atomic bomb, with many British scientists involved in the Manhattan Project. Importantly, under the wartime Quebec agreement, Britain surrendered the right to veto American use of atomic weapons. Agreements at the Quebec Conference in 1943, at Hyde Park in September 1944 and in Washington in 1945 promised continuing nuclear cooperation after the end of the war.
The McMahon Act of 1946 stipulated that the US would not share information concerning atomic weapons. This was a bitter disappointment to the British government, and as a direct result Attlee's government initiated its own atomic weapons programme in January 1947. This led to the first test at Monte Bello in October 1952.
During the late 1940s and early 1950s the British hoped that pursuing their own atomic weapon programme would force the US to revise the McMahon Act. Instead America used atomic energy information (not atomic weapons information) to bargain for increased access to uranium stocks, obtaining a large quota in the Congo. Negotiations during 1947/8 produced an agreement known as the modus vivendi that allowed exchange of atomic information unrelated to weapons.
Only a limited flow of atomic information arrived in Britain, which was a continual source of disappointment and tension. Cold War tensions mounted after the Korean conflict, which brought an increase in Anglo-American nuclear weapons cooperation. Meetings between Churchill and Dwight D. Eisenhower resulted in the American Atomic Energy Act of 1954, enabling the sharing of information regarding external characteristics of atomic weapons.
In the 1950s further agreements were signed between Britain and the US. An agreement in June 1956 provided Britain with information about nuclear submarine propulsion systems that later enabled its nuclear submarine programme. Despite advances in nuclear submarine propulsion, Britain's deterrent was based on RAF jet bombers - the V-bomber force. The British tested their first thermonuclear weapon (the hydrogen bomb) in May 1957. The birth of the nuclear armed ballistic missile made the RAF's V-bomber force out-of-date and expensive.
The fallout from the Suez Crisis forced Britain to reassess defence. A British-designed medium range ballistic missile, Blue Streak, was not expected to be able to replace the V-bomber force until the mid 1960s. At the same time, the resignation of Anthony Eden as Prime Minister and his replacement by Harold Macmillan allowed for new thinking on strategic policy. Macmillan was keen to develop the concept of Anglo-American nuclear interdependence. In March 1957 Macmillan met Eisenhower in Bermuda and agreed to the deployment of Thor IRBMs in Britain. The British H-bomb test provided a valuable bargaining tool with the Americans and the new Atomic Energy Act of 1958 overturned the McMahon Act. The new act featured an Anglo-American agreement to share information on nuclear weapons, fissile material and the purchase of nuclear materials.