A previously unseen diary has revealed details of a series of secret meetings during the Second World War, which led to the creation of the UK-US intelligence-sharing alliance which still exists today.
GCHQ has released entries from the private diary of its first director, showing how the relationship was first formed, including secret meetings at Bletchley Park near Milton Keynes.
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 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.
In a joint statement, current GCHQ head Jeremy Fleming and the director of the US National Security Agency, General Paul Nakasone, said that in the modern digital world is constantly evolving. Adding threats don't respect international borders and global partnerships are key to our security and economic prosperity and none more so than the one between our two countries