The successful development of British thermonuclear weapons came at an opportune moment to renew negotiations with the Americans. The Soviet Union's launch of Sputnik 1, the world's first artificial satellite, on 4 October 1957, came as a tremendous shock to the American public, which had trusted that American technological superiority ensured their invulnerability. Suddenly, there was now incontrovertible proof that in at least some areas, the Soviet Union was actually ahead. In the widespread calls for action in response to the Sputnik crisis, officials in the United States and Britain seized an opportunity to mend their relationship. At the suggestion of Harold Caccia, the British Ambassador to the United States, Macmillan wrote to Eisenhower on 10 October to urge for both countries pool their resources, as Macmillan put it, to meet the Soviet challenge on every front, "military, economic and political".