In November 1962 the British and French governments signed an irreversible agreement to build together the Western world's first civil supersonic aeroplane: Concorde. This article explores the background to the agreement, looking in particular at why Britain and France co-operated with each other rather than with the United States or West Germany, the other possible partners. The central argument is that the agreement was driven not by technological convergence in the aircraft industry but by a weakening of Britain's position in 1961 caused by American unwillingness to partner Britain in the supersonic adventure and Britain's decision to apply for membership of the Common Market. The French government was then able to exploit that weakness in order to acquire technological expertise particularly in the field of aircraft engine construction.