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, a previously unseen diary has revealed.
GCHQ (Government Communications Headquarters) has released entries from the private diary of its first director, showing how the relationship was first formed, which later became Five Eyes.
As the threat from the Nazis was replaced by a new one from the Soviet Union, the 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 United States.
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, from 10 February 1941, Commander Alastair Denniston, then head of GCHQ's predecessor the Government Code & Cypher School, wrote: "The Ys [Yanks] are coming!"
The entry was in reference to the Sinkov Mission when a group of Americans secretly travelled across the Atlantic to Bletchley Park, Buckinghamshire, where Allied code-breakers operated.
Commander Denniston famously told his assistant Barbara Abernethy of their pending arrival: "There are going to be four Americans who are coming to see me at 12 o'clock tonight.
"I require you to come in with the sherry. You are not to tell anybody who they are or what they will be doing."
The visit was a success and intelligence was shared, including Britain’s greatest secret – the Bombe machine, designed to break the German Enigma code.
Denniston's diary 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 in New York.
In a joint statement, current GCHQ head Jeremy Fleming and the director of the US National Security Agency, General Paul Nakasone, said the partnership "helps us equip our leaders with the information they need" and "ultimately makes the UK and US safer".
"This alliance defines how we share communication, translation, analysis and code-breaking information, and has helped protect our countries and allies for decades.
"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. We celebrate those 75 years and we look forward to the future together," he added.
Cover image: The former diary from Commander Alastair Denniston, the first head of GCHQ (Picture: GCHQ).