The day after the attacks of September 11, 2001, the remains of most of the 2,977 victims still lay buried deep in the wreckage of the World Trade Centre, at the western wall of Pentagon and in a Pennsylvania field. But the White House was already zeroing in on an old nemesis: Iraq's Saddam Hussein.
There was Afghanistan to deal with first, of course. "Start with [Osama] bin Laden, which Americans expect," the president George W Bush told his top military, intelligence and foreign policy officials the day after the attacks, according to an authoritative account by American journalist and author Bob Woodward. "And then if we succeed, we've struck a huge blow and can move forward."
The US military's top civilian official, Donald Rumsfeld, then asked whether the attacks did not present an "opportunity" to strike against Iraq, officials present at the meeting later told Woodward.
The wheels of war that were set in motion that day, even as the smoke from the previous day's attacks was still clearing, culminated 16 months later in the US-led invasion of Iraq.
The onslaught succeeded in removing a cruel tyrant. It also produced "one of the great horror stories of the post-World War II era", believes Juan Cole, a professor of Middle East history at the University of Michigan in Ann Arbor.
Finding Saddam's alleged weapons of mass destruction would be a "slam dunk", the head of the Central Intelligence Agency, George Tenet, told President Bush. Liberating Iraq would be a "cakewalk", declared Kenneth Adelman, a former top aide to Mr Rumsfeld.
But the chaotic aftermath of "Operation Iraqi Freedom" gave Al Qaeda a breath of life after it had been mostly vanquished in Afghanistan, and it boosted Iran's regional power. It triggered a furious sectarian war between Iraq's Sunnis and Shiites and helped unleash the demons of sectarianism that still afflict the region to this day.
All told, 108,624 Iraqi civilians have been killed since the 2003 invasion, most of them in sectarian fighting, said a study published this week in the British medical journal The Lancet. Between 2005 and 2008 alone, an average of 60 Iraqis were being killed each day, a US government study later concluded.
By contrast, estimates of how many Iraqis were executed or otherwise "disappeared" during Saddam's 24-year regime range from 300,000 to 800,000.
After the US invasion, no weapons of mass destruction were ever found, and an independent commission established by the US Congress concluded in 2004 that Iraq had nothing to do with the 9/11 attacks.
By then, however, long-lasting damage had been done, both to Iraq and Washington's own interests.
In a particularly searing indictment of the US road to war in Iraq, the former head of MI5, the United Kingdom's counter-intelligence and security agency, last week called the invasion a "distraction in pursuit of Al Qaeda".
In a lecture delivered in London, Lady Eliza Manningham-Buller described Saddam as a "ruthless dictator" but said "neither he nor his regime had anything to do with 9/11". The invasion, she added, "provided an arena for jihad", spurring on UK citizens to resort to terror.
Nearly eight years after Saddam's capture by US soldiers, critics of the war are unwilling to give Mr Bush credit for Iraq's fragile democracy.
Patrick Seale, a veteran Middle East analyst and author, said: "It isn't working very well and the violence certainly hasn't stopped. And Iraq is no longer a united country - it's a loose federation."
Many Iraqis themselves refuse to be drawn into the question over whether their country is better or worse for the invasion.
"It's not a question of better. Iraq is a failed country. Most Iraqis are of course pleased Saddam has gone but they're also not happy with the present regime," said one Iraqi businessman.
Regionally, rather than advancing the cause of democracy, the Iraq war "stalled or reversed the momentum" towards liberalisation, the Rand Corporation said in a study last year. Autocratic regimes calculated that the US's "distraction in Iraq and its focus on containing Iran" gave them a reprieve from "the post-9/11 agenda for political reform".
Perhaps the biggest unintended consequence of the post-9/11 push against Saddam Hussein was to tilt the regional balance of power in favour of Shiite Iran.
After forcing from power Iran's other main regional foe, the extremist Sunni Taliban regime in Afghanistan, the US invasion cleared the way for Iraq's majority Shiites to hold sway in Baghdad. This fuelled the perception among US-backed, Sunni-dominated governments in the region that Iran - and by extension, Shiism - was on the winning side.
Beyond the sheer cost in blood and treasure in Iraq, the once shining hopes of the American neoconservatives who drove post-9/11 US policy and hoped to remodel the Middle East have been extinguished, Mr Seale said.
They pushed for the post-9/11 invasion of Iraq in the hope of dealing a "death blow to Palestinian nationalism, Islamic radicalism and Arab nationalism".
After quickly dispatching with Iraq, "it would have been Syria next, then Iran and they even talked about 'reforming' Saudi Arabia and Egypt". It was, Mr Seale said, "a foolish geopolitical fantasy".
That the US is now preparing to pull out of Iraq "when it is by no means stable" is evidence that the Bush administration failed in nearly all its stated war aims, said Gerald Butt, a former Middle East correspondent for the BBC and author of several books about the region.
Today, about 46,000 American soldiers are in Iraq, down from 140,000 a couple of years ago. Barack Obama, the US president, has pledged to bring them home by the end of the year, but Washington is as divided as Baghdad on the controversial question of whether some should remain behind on a post-2011 training mission.
Security remains a key issue in Iraq, with institutions still weak and too many Iraqis still preferring conduct their politics violently. To a dismaying degree, fear still reigns.
Another well-known scholar of Middle East politics said Iraq's democratic procedures are not robust enough to rely on when the Americans leave.
"I fear it will be the bullet, not the ballot, that has the final say in Iraq," said Larbi Sadiki, a senior lecturer at the University of Exeter in England.