Over the past century, certain initials have become world-famous, with resonances either good, like the BBC, or loathesome: KGB, SS, IRA. Between those two extremes, the CIA is now at least as celebrated as any of the others. According to Tim Weiner, in the summer of 1953, Winston Churchill had never heard of it, but soon there would be few prime ministers, presidents or dictators who were unfamiliar with what the Central Intelligence Agency was, or what it did.

Created in 1946 out of the remnants of the wartime Office of Strategic Services (renowned for its derring-do and its ineptitude), the CIA expanded rapidly as the cold war developed. Harry Truman said that he never wanted it “to act as a spy organisation” rather than as a bureau collecting and collating intelligence in the broadest sense, and was even worried that he might be creating some kind of Gestapo. But then one of the themes of Weiner’s book is the awkward relationship between successive presidents and an overmighty agency. His very title is taken from Eisenhower, who complained that he had failed “to make sense of the agency”, and feared that he would leave his successor “a legacy of ashes”.

Towards the end of this long and well-informed if sometimes breathless book, Weiner observes that during the cold war the left damned the CIA for what it was doing, whereas in recent years the right has damned it for what it was failing to do. They both had a point. Few soothsayers have had such a poor record of prediction as the CIA: it didn’t foresee the Russian atomic bomb or the entry of China into the Korean war, and 50 years later, it had no inkling of the September 11 attacks.

A New York Times reporter, Weiner has researched the subject more fully than anyone before, making use of many hitherto unexplored archives. He portrays the senior officers of the CIA as Ivy League amateurs, too many of them neurotic, dipsomaniac, sexually eccentric or plain barking. Again and again, the CIA recklessly occasioned the demise of its operatives rather than its enemies, as enthusiasts were sent off to certain death in east Europe or China: “We’d drop these people in,” one CIA man said ruefully, “and we’d never hear from them again.”

Then there was the appalling business of Hungary in 1956, when Radio Free Europe, run by the CIA, incited the people of Budapest to rise up against their Soviet rulers in the expectation of American support that never came. Under President Kennedy and his brother Robert (who comes out of this book very badly) the agency conspired to kill Fidel Castro or to destroy his regime, but he is still there after nearly 50 years in power. And leave aside what the Vietnamese or Chileans wanted. Had the Americans voted for the assassination of Ngo Dinh Diem (this after “the agency created South Vietnam’s political parties, trained its secret police, made its popular movies, and printed and peddled an astrological magazine predicting that the stars were in Diem’s favour”), or the putsch that installed General Pinochet’s dictatorship?

What Weiner neglects is Maitland’s first rule of history: always remember that events now far in the past were once in the future. Even the generally maligned Tehran coup of 1953 did, in fact, keep Soviet Russia from reaching the Indian ocean, its avowed purpose. When Kim Roosevelt and his CIA and MI6 comrades deposed Mohammed Mosaddeq and installed the Shah, they doubtless did not foresee the rise of militant Islamism a generation later, but who did?

Maybe the real problem was that the CIA became too mighty, too irresponsible, and simply too big, as it acquired private armies, newspapers, trade unions, airlines. And yet with all this – absurdly enough in view of Truman’s initial apprehensions – the most powerful country on earth “has failed to create a first-rate spy service”. By 1999, when duff gen led to the American aircraft bombing the Chinese embassy in Belgrade, the sour Nato gag was that the initials stood for “Can’t Identify Anything”.

Hovering over the book is a question that Weiner doesn’t properly address. Since 1941, America has became the only global hyper-power. But does Weiner (do Americans) now want any active foreign policy at all? His book could well be used to make a case for old-fashioned isolationism, though that’s unlikely to be his intention.

At the end he is simply wrong to blame the Iraq fiasco on the CIA alone, and George Tenet in particular with his “slam dunk” assurance that Saddam Hussein had weapons of mass destruction. In defiance of the rule that intelligence must always drive policy and not the other way round, the decision to invade Iraq was taken by the Bush administration and its British satellite. The CIA and its colleagues in London were then enlisted less as intelligence than advertising agencies, to make a case that would justify that decision.

Even so, the book tells a salutary story. Back from Camp David, our new prime minister should find it most instructive reading.

LEGACY OF ASHES: The History of the CIA by Tim Weiner
Allen Lane £25 pp704
Buy the book here at the offer price of £22.50 (inc p&p)

Explore Books

• Book Extracts

• Book Reviews

• Books Group

Times Recommends

• The Gone-Away World by Nick Harkaway
• Burning is too good for them
• Strange Fruit: Why Both Sides Are Wrong in the Race Debate by Kenan Malik