In America they speak of the "lost decade". The moment it began is obvious: the morning of September 11, 2001, when the world's lone superpower fell victim to the most devastating terrorist attack of modern times.
Its end, however, is harder to date.
One answer is May 1 this year, when a team of Navy Seals tracked down and killed Osama bin Laden at his hideaway in Pakistan. A circle was complete; after almost 10 years of frustration, false leads and continuous war, the master planner of 9/11 had finally paid for his crime. But look at it another way and the answer is not so obvious.
In these 10 years America has lost much, in terms of lives, treasure and reputation. Most of all, perhaps, it has lost its illusions.
One, that its home territory was invulnerable, beyond the reach of hostile foreigners, vanished on that terrible Tuesday morning. But a decade on, another no less cherished illusion has disappeared as well: the certainty that whatever happened in the world beyond, America was a place of infinite opportunity and ever-growing prosperity.
Formally, of course, the anniversary of 9/11 is today NZT. But when one era closes, another begins.
In a sense, this lost decade ended on Friday, when President Barack Obama presented Congress with his plan to prevent the US economy from slipping back into the worst economic recession since the 1930s. It was a call to arms that mirrored President George W Bush's in the same place when he declared his "war on terror". As rarely before, an American President acknowledged bitter economic reality. It has taken a decade.
The attacks of September 11, 2001 changed everything. If the 20th century in reality ended on Christmas Day 1991, at the moment the Soviet Union passed into history, the 21st century began only on that perfect autumn morning along the US mid-Atlantic seaboard as apocalypse arrived.
Even now, our minds do not have space to accommodate simultaneously the tragedies of that day: the destruction, the 3000 lost lives, the shattered families, the heroism of New York's firefighters and the bravery of the passengers on UA93, who sacrificed themselves to prevent a direct hit on the US capital. Even now, those images of planes ripping like darts into gleaming skyscrapers amid a giant burst of yellow, orange and black seem surreal.
And then there's everything that flowed from 9/11: the wars, the changes in a wounded America's attitude to the world, either "with us or against us", as well as in foreign attitudes towards America, and the later terrorist attacks in Bali, London, Madrid and elsewhere.
Could it have been prevented? Perhaps, but not without a huge slice of luck. The report on the attacks, published here in 2004 by the non-partisan 9/11 Commission, identifies "four kinds of failure: in imagination, policy, capabilities and management".
The management shortcomings consisted mainly in the shameful lack of co-operation between the FBI and the CIA, the two main counter-terrorism agencies, and the slow response of FBI headquarters to warnings from its agents in the field.
The Pentagon was still relying on Cold War thinking and Cold War weapons. Moreover, the country's defences faced outward, to counter any threat from abroad, not internal ones. On the policy front it was a similar story: before 9/11, the biggest foreign preoccupations were the Balkans, Saudi Arabia and the Middle East conflict. The US was well aware of al-Qaeda - President Bill Clinton had sent cruise missiles against its bases in 1998. Beyond that though, what was to be done: invade Afghanistan?
Today, much has changed. The FBI and the CIA seem to be working more smoothly together, though some tensions remain, and America has a new bureaucratic behemoth, the Department of Homeland Security. The efforts of the latter may be seen in the numbing security procedures at airports.
Counter-insurgency and asymmetric warfare are now core military missions. Unmanned drones are more widely used than ever; elite special forces have been greatly enlarged.
Despite every prediction to the contrary in the aftermath of 9/11, there have been no more attacks on mainland America. It has taken 10 years, but the US may be close to a knockout blow against al-Qaeda, at least the original al-Qaeda that bin Laden ran.
Even with the full advantage of hindsight, it is still hard to grasp how a group of fanatics and social misfits, numbering half the size of an army platoon, in an operation costing just US$500,000 and run from one of the poorest, most backward corners of the earth, could have wreaked such mayhem on the most powerful country in history. .
According to one academic study, the final bill for this age of wars, the longest in US history, could run to US$4 trillion.
The "war on terror" and all that flowed from it also did shocking damage to America's reputation. Even the election of the first black President who, having spent part of his childhood in the Muslim world, was peculiarly well-equipped to see the US as others see it, has failed to expunge the image of arrogance, unilateralism and sheer ignorance created by his predecessor.
Contrary to his initial promises, Guantanamo Bay remains open for business, and terrorist suspects will continue to be tried (if they are tried at all) in military courts. Drone attacks are more frequent than ever.
At home, the panic of a decade ago has subsided. Americans increasingly accept that terrorism is a part of the world we live in. But the trauma of that terrible day lingers.
September 11 does bear some blame for the severity of the crisis and lack of options open to Obama. Bush chose to pay for his wars with borrowed money, adding to the debt that ties his successor's hands. But you can't blame bin Laden for the government's mania to deregulate.
Nor was bin Laden responsible for the years of excessively low interest rates set by the Federal Reserve that fed the reckless mortgage lending that led to the sub-prime crisis, or for the equally reckless behaviour of the banks. Nor has he anything to do with America's chronic inability to live within its means.
And in politics too, the story is the same. Partisanship and endless squabbling have reduced the system to near-terminal dysfunction.
After 9/11, Bush - his popularity at a stratospheric 90 per cent - could have used the moment to do bold things, unimaginable in normal times, and which might have mitigated the economic crisis that lay ahead. He could have reversed some of his tax cuts to pay for his wars; he could have imposed a petrol tax to raise revenue and reduce America's crippling dependency on oil. But the "war on terror" eclipsed all else. In its name the US would spend its blood and treasure to secure a safer world, in the mistaken belief that nothing could not be achieved by the might of American arms and the imposition of Western-style democracy. All the while, China and other rising industrial powers moved quietly but steadily forward. They captured American markets and American jobs, and bought vast quantities of US debt to enable the country to continue to consume more than it produced.
From Washington, presidents have continued to peddle the doctrine of "American exceptionalism". Their country was different from other countries, Bush told them, it had a special place in the world. It could make its own rules, and everything would come right in the end. In the immediate aftermath of 9/11, such language was a necessary morale-booster for a traumatised nation. These days it is wilful evasion of reality. To Obama's credit, his speech on Friday, acknowledging that the US faced a protracted "national crisis", contained remarkably little that sugared the pill of truth.
Ever since the Depression, a prime tenet of "the American Dream" has been that each generation will be better off than the one before it. Until the turn of the century, that was true. But since 2001 the poverty rate has risen, the median income has declined, the number of people without health insurance has grown and the gap between rich and poor is wider than at any time since the 1929 Wall St Crash.
Most dispiriting of all is unemployment. The headline figure is 9 per cent.
A decade after the horrors of 9/11, the world's lone superpower is learning anew one of history's constants, that economic crises can be even more intractable than wars. Wars unite, and what would the country not give to rediscover the sense of unity spawned in September 2001.