The Big Brother on Menwith Hill
It started with Enigma
The Quadripartite Agreement of 1947
In 1947, four English-speaking countries - the United States, the United Kingdom, Australia (together with New Zealand) and Canada - signed a secret treaty (some sources suggest that even Prime Ministers were kept unaware) about sharing intelligence data. The treaty has also been known as UKUSA (UK plus USA). As such, it was beneficial for the smaller (in terms of intelligence resources) partners who did not have comparable intelligence systems with the UK and the US. The treaty allowed sharing of raw intelligence data, but analysis and conclusions were made separately by each state, which sometimes led to contradictory results based on the same data. Under the terms of the agreement, the five nations carved up the earth into five spheres of influence, and each country was assigned particular signals intelligence targets (e.g. Britain was responsible for intercepting the Chinese, through its Hong Kong listening post, while the US was given other responsibilities to cover from its listening posts in Taiwan, Japan and Korea).
While the details of the treaty remain largely unknown to the public, it has been suggested that the infamous bypass system for the gentlemanly ban on domestic spying (a state would not spy after its own subjects at home) was born already with this treaty. A former employee of Canadian CSE (Communication Security Establishment) Mike Frost wrote a book titled Spyworld in 1994 which also has an account of Canadian agents snooping after Brits (see a review of the book by Bob Leonard.