"If there was one constant to any account of postwar British foreign policy, it is the centrality of the United States. In the past 20 years, the importance and role of the intelligence relationship that underpins this factor have become more prevalent. Yet, attention is often focused on specific aspects. The 1946 UK-USA Agreement, for instance, which provided the backbone to the sharing of signals intelligence to this day, is often cited as the central pillar of the special intelligence relationship. Similarly, in episodic instances the covert relationship is cited, with notable examples including the restoration to power of the shah of Iran in 1953 and the running of agents like Oleg Penkovksy."
United States. Central Intelligence Agency: http://www.cia.gov/
Studies in Intelligence (Extracts, June 2015), vol.59 no.2