Following the success of the Grapple X/Round C experiment and joining the club of thermonuclear nations (the US and the USSR at the time), Britain launched negotiations with the US to join the US in a treaty situation in which both nations could share information and material to design, test and maintain their nuclear weapons. This effort culminated in the 1958 US–UK Mutual Defence Agreement. One of the results of that treaty was that Britain was allowed to use United States' Nevada Test Site for testing their designs and ideas, and received full support from the personnel there, in exchange for the data "take" from the experiment, a mutual condition. In effect the NTS became Britain's test ground, subject only to advanced planning and integrating their testing into that of the United Stat