A series of secret meetings during the Second World War led to the creation of the UK-US intelligence-sharing alliance that still exists today, previously unseen diary details reveal.
GCHQ has released entries from the private diary of its first director, showing how the relationship was first formed.
As the threat from the Nazis was replaced by a new one from the Soviet Union, an agreement formed the basis for co-operation in the Cold War.
Signed in Washington in March 1946, the document sets out post-war arrangement for sharing intelligence between the UK and the US.
Australia, Canada and New Zealand joined in the following 10 years, making up the Five Eyes alliance.
In a statement marking the pact’s 75th anniversary, GCHQ and the US National Security Agency said it had made the UK and US “safer” decades later.
In one diary entry on February 10 1941, Commander Alastair Denniston, then head of GCHQ’s predecessor the Government Code & Cypher School, wrote: “The Ys (Yanks) are coming!”
He was referring to the Sinkov Mission, when a group of Americans made a secret journey across the Atlantic to Bletchley Park, Buckinghamshire, where Allied code-breakers operated.
The visit was a success and intelligence was shared, including Britain’s greatest secret – the Bombe machine, designed by Alan Turing and Gordon Welchman to break the German Enigma cipher.
Denniston’s entries show he met his US counterpart William Friedman, chief cryptanalyst in the Signals Intelligence Service, across the Atlantic.
There are also references to at least two other meetings across New York.
In a joint statement, current GCHQ head Jeremy Fleming and the director of the US National Security Agency, General Paul Nakasone, said: “This year, we celebrate the 75th anniversary of the formal partnership between the UK GCHQ and the US National Security Agency.
“This alliance defines how we share communication, translation, analysis and code-breaking information, and has helped protect our countries and allies for decades.
“The modern digital world is constantly evolving. Threats don’t respect international borders. Global partnerships are key to our security and economic prosperity – and none more so than the one between our two countries.
“For 75 years, this extraordinary partnership has enabled us to evolve and learn from each other. It helps us equip our leaders with the information they need. And it ultimately makes the UK and US safer.
“We celebrate those 75 years and we look forward to the future together.”