David Goddard / Getty Images The Government Communications Headquarters building in Cheltenham, England

For drama, it couldn't quite compete with the arrests on Monday, June 28, by the U.S. Department of Justice of 10 people charged with spying  with eight of them allegedly carrying out "long-term, deep-cover assignments for the Russian government." But for armchair spy catchers and amateur historians, last Friday's revelations were of even greater historical significance. Britain's National Archives and the U.S. National Security Agency (NSA) made public one of the most important documents in the history of the Cold War: the seven-page UKUSA Agreement, a secret pact that, since 1946, has allowed the two countries to share intelligence with each other. The agreement, whose details were released alongside examples of the kind of information that was passed back and forth, fills in significant gaps in the history of the Cold War and reveals one of the foundations of the special relationship the U.K. and the U.S. still hold dear.

According to Christopher Andrew, official historian of the U.K.'s internal security service MI5, it is thanks to UKUSA (pronounced yew-kew-zah) and the sharing of signals intelligence, or SIGINT  information gathered through eavesdropping, phone-tapping and radio intercepts  that since World War II, "Britain and the U.S. have shared more secrets than any two independent powers in the history of the world." While histories of World War II inevitably mention high-level ultra-classified decrypts  and how a message from Hitler to his generals could arrive on Churchill's desk before it reached the field  studies of the Cold War almost wholly fail to mention the vital importance of SIGINT. President Harry Truman understood how valuable it was to know more than the other guy, and so, despite closing down the Office of Strategic Services, the precursor to the CIA, in September 1945 on the grounds that (as Secretary of State Henry Stimson had once put it) "gentlemen do not read other gentlemen's mail," he agreed to a code-breaking alliance with the U.K.  a far-sighted decision in the very first days of the Cold War. The British-U.S. Communication Agreement, later renamed UKUSA, was so secret that its existence wasn't even acknowledged until 2005. (Canada, Australia and New Zealand were inducted into the alliance several years after the agreement was first signed.)

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Along with UKUSA, the National Archives has also released a selection of more than 3,000 intelligence reports from Government Communications Headquarters (GCHQ)  Britain's version of the NSA  that were collected from 1946 to 1949. Documenting coded messages between Soviets at all levels of society, primarily midlevel officials, the reports are simultaneously impersonal and deeply revealing. They provide a portrait of a country struggling with a Stalinist bureaucracy and suffering from the loss of more than 26 million citizens in what is still the most expensive war ever fought. "The small print forms a kind of pointillist picture, individual human stories which add up to the whole," says Keith Jeffery, whose official history of the British Secret Service, or MI6, comes out in September. The GCHQ intercepts detail hobbled public transportation, slashed subsidies and pensions for workers and the abolition of free communal services. When transportation problems leave workers in Siberia without food and clothing, authorities send a fleet of new cars. "See that they are not stolen from you," warns the Moscow controller. "Somebody will seize them under the cloak of requisitioning." (See 10 things to do in Moscow.)

Some intercepts make very plain the culture of autocracy and censorship of Stalinist Russia. A directive from the "Chief Directorate for the Control of Entertainments and Repertory" forbids the performance of several popular "pseudofolk songs that are inartistic and trivial." Two clandestine messages, sent between private citizens, read the most as something out of a spy novel, with one saying, "Sell the coat immediately, refrain from buying" and the other, "Don't sell the furs  details by letter." A note on the report mentions that these are "similar to many private telegrams exchanged between Soviet citizens"  it seems the officials were not the only ones talking in code.

The most telling exchanges, though, are those that were of little use to military intelligence but are now invaluable to our understanding of history. In one, a Polish citizen begs the chief directorate of prison camps for the release of her daughter, stricken with tuberculosis, from internment in central Russia. In a telephone call in March 1949, a woman preparing to go on holiday alone tells her husband, an official at the Ministry of State Security, "I am afraid of leaving the kids here. What about a war, all of a sudden?" And a heartbreaking message addressed to the widow of Politburo member Sergo Ordzhonikidze, once a close friend of Soviet dictator Josef Stalin, simply reads, "We live in poor circumstances. The child is sick. Please help."

While UKUSA is still technically in effect, the assumption that it still allows uninhibited exchanging of information between Britain and the U.S. is "one of the great myths" of intelligence, says Richard Aldrich, author of GCHQ, a history of the supersecretive organization. In the modern world, he explains, intelligence sharing does not extend to the Middle East, for example, where competing ideologies and interests have led to a doubling-up of intelligence gathering as both the U.S. and the U.K. seek information they are unwilling to share. "The first three decades of the Cold War were the glory days of UKUSA," says Aldrich. In a world where one great secret  the existence of UKUSA  was revealed only after 60 years, perhaps the special relationship is not as robust as it once was. But with the agreement and intelligence reports now available to the public, history fans can at least roll back the years and play spy.

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