On the same spring day in 1946 that Winston Churchill made a speech coining the phrases “special relationship” and “iron curtain”, another historic event that would help to shape the next 75 years took place. A secret pact was signed between the UK and the US, a formal agreement to share intelligence in order to combat the Soviet threat.

In time, as Canada, Australia and New Zealand signed on, too, that agreement would become known as Five Eyes – although it was not until 2010 that the alliance was made public. Five Eyes, much like the UK’s permanent membership of the United Nations Security Council and its leadership role in Nato, has allowed Britain to feel, post-Brexit, like it still has a seat at the top table. But, perhaps understandably for a secret alliance of intelligence agencies, little about its inner workings have ever been known.

Richard Kerbaj, a former security correspondent for the Sunday Times and documentary-maker, has made a decent stab at lifting the curtain. He has persuaded many of those involved in Five Eyes to speak to him, and delved into national archives of all five nations to piece together an understandably partial history.

It is a tale that reveals an alliance marred by mistrust, mistakes and misjudgments, one that likes to see itself as responsible for keeping its nations safe but has, at times, not only failed in that endeavour but has also contributed to global insecurity.

Until the end of the cold war, the five nations were united in their attempts to defeat the Soviet Union. Part of that involved rooting out Soviet spies or turning Russian diplomats. This is the work that the Five Eyes has been happy to reveal. It has been less keen on shining a light on the darker role it has played.

Intelligence agencies may like to see themselves as defensive outfits – preventing attacks, not carrying them out themselves. But over the past 70 years, British and American intelligence agencies have been responsible for a series of aggressive moves that have destabilised the Middle East, contributing to many of the geopolitical problems that still exist today.

In the late 1950s, CIA chief Allen Dulles orchestrated the overthrow of a series of democratically elected governments from Iran to Guatemala. In Syria he oversaw “a series of conspiracies” to overthrow the government for committing the crime of refusing to join a western-led military alliance. As Kerbaj points out, “the CIA’s botched operation magnified the already growing anti-western sentiment in the Middle East”.

In the early 1980s, the CIA and MI6 worked together to fund, support and arm the mujahideen in Afghanistan in order to help them defeat the Soviet Union. One of the CIA’s “greatest recipients of funding and arms”, Kerbaj notes, was the leader of the Haqqani network, a group the director of national intelligence now deems to be a terrorist network whose leaders are on the US’s most wanted list.

And then there is Iraq. This was a war supposedly based on intelligence, namely that Saddam Hussein had weapons of mass destruction, that he was prepared to use them, and that he was willing to share them with terrorist groups such as al-Qaida. None of these theories were true. George W Bush and Tony Blair may have sold the war, but the material they used was created by members of Five Eyes.

Not only that, those intelligence agencies were also responsible for some of the most egregious human rights abuses carried out by western powers. Torture was technically illegal, so they instead kidnapped suspected terrorists, flew them to countries with less stringent rules, allowed them to carry out the torture and even provided lists of questions to be asked once the suspects were deemed malleable. It was an American-led operation – Canadian, British and Australian citizens were sometimes the victims – but all members of Five Eyes knew it was happening. Indeed, there were numerous cases where British officials were accused of involvement.

The programme, named “extraordinary rendition” rather than the more accurate “kidnap and torture”, would, as Kerbaj notes, “ultimately haunt the legacy of the Five Eyes”.

The mistrust that has dogged the alliance from the start still exists today. Seventeen British nationals and citizens were held at Guantánamo but it was two years before the US agreed to release just five of them. Kerbaj reveals that Peter Clarke, former head of Scotland Yard’s counter-terrorism command, was urged by the US to imprison them on their return, something Clarke immediately dismissed. “None of the material they provided us with would have been admissible in court,” Clarke told Kerbaj.

Arguably worse is the case of Shamima Begum, who left London in 2015, aged 15, to join Islamic State, and who has now been stripped of her British citizenship. Kerbaj reveals that Begum was smuggled into Syria by a Canadian spy, a fact the Canadians initially withheld from their supposed ally. Both the Canadian and British governments decided to keep this a secret. Begum’s lawyers hope this revelation will help her win the appeal against the decision to remove her citizenship, which takes place next month.

Intelligence agencies tell us they keep us safe. And perhaps they do. But the stories Kerbaj tells reveal a different truth. From 2001 onwards, this is a story of failure – of missing warnings that could have prevented atrocities, of misusing intelligence to start a war, and of using its almost untrammelled power to terrorise its own citizens.

Spot a problem in the Middle East and Five Eyes either didn’t see it coming, or inadvertently gave it a helping hand.

And yet Kerbaj, oddly, comes to a different conclusion. After 15 chapters outlining the disasters and revealing the outrages, he ends with a parade of 14 former spy chiefs and prime ministers explaining why Five Eyes matters. No critics are quoted, nor does Kerbaj himself offer any alternative view. What he has gained in access he has lost in analysis.

After 300 plus pages of scandal, Kerbaj blandly concludes: “The alliance remains vital in attempting to foresee and combat future threats.” It is a bizarre end to an, at times, brilliant book.