"If you're asking, are there al-Qaeda in Iraq, the answer is yes, there are. It's a fact, yes." Donald Rumsfeld, US Secretary of Defence, August 2002
It was one of the key American justifications for the Iraq war. But the theory that al-Qaeda was present in Saddam-era Iraq, much cited by the Bush administration in the run-up to the invasion, has been undermined by the content of secret US military documents.
The files contain only half a dozen references to the group for the whole of 2004, the year records begin. But under the leadership of Abu Musab al-Zarqawi, a Jordanian who had met Osama bin Laden while fighting the Soviet occupation of Afghanistan, the classified reports show that al-Qaeda established itself as major player in the carnage as the conflict wore on.
The infrequency of al-Qaeda-related files early in the war suggest that US officials were wrong when they accused Iraq of harbouring the group's fighters in the years prior to the invasion. Instead, the narrative that emerges from the classified reports indicates that the US presence itself was what attracted them to the country.
Experts have always doubted the existence of a link between the secular Baathist government of Saddam Hussein and al-Qaeda.
Speaking at the British inquiry into the Iraq War in 2005, Eliza Manningham-Buller, the UK's spy chief at the time of the invasion, said that even US intelligence officials doubted a connection between Iraq and the group.
"There was no credible intelligence to suggest that connection and that was the judgement, I might say, of the CIA," she told the inquiry. "It was not a judgement that found favour with some parts of the American machine."
"It is why Donald Rumsfeld started an alternative intelligence unit in the Pentagon to seek an alternative judgement," Manningham-Buller, who was a frequent visitor Washington in the run-up to the war, said.
"Saddam Hussein had nothing to do with 9/11 and I have never seen anything to make me change my mind."
When al-Qaeda did arrive in Iraq, it wreaked havoc in the country. Its fighters announced their presence with a ruthless campaign of kidnappings and televised beheadings of foreigners starting in late 2004.
With a supply of sympathetic fighters willing to enter Iraq and fight the US occupation – the documents hold thousands of references to "foreign fighters" - it then launched a series of suicide bombings, utilising a flow of foreign volunteers that peaked in 2005.
As the situation in Iraq evolved, so did the group's tactics. Perhaps realising that sectarian fighting would delegitimise the US presence in the country, al-Qaeda did its utmost to sow sedition between Iraq's religious communities.
In February 2006, the group plunged Iraq into the bloodiest phase of the conflict by bombing the al-Askari mosque, one of Shia Islam's holiest sites.
The attack triggered a wave of revenge attacks from Iranian-backed Shia militias, forcing thousands of Iraqis to flee their homes to escape the sectarian violence.
But the attack, and the violence it sparked, also tarnished al-Qaeda's reputation. For many potential supporters, opposing the US occupation of Iraq was one thing, but efforts to plunge the country into civil war were quite another.
Four months after the Samarra bombing, US forces discovered Zarqawi's location and launched an air raid, killing him. But his deadly ideology would live on.
In 2007, as the Washington ordered a troop "surge" to Iraq, Sunni militias that had previously tolerated al-Qaeda were persuaded to turn against the foreign fighters in their midst.
Al-Qaeda in Iraq did not give up without a fight. The leaked documents reveal that the number of attacks by the group and their allies went up during 2007.
The more recent attacks seem to employ increasingly desperate tactics, recruiting children, women and the disabled to serve as suicide bombers. One shocking report, from April of 2008, describes how a bombing was carried out by a "16-17 year old male, who looked mentally-retarded and had facial features of a person with Down syndrome".
But as Al-Qaeda's support dried up, so did its money, and fundraising appears to have become a serious issue for the fighters. When a Christian archbishop was kidnapped in northern Iraq in 2008, Al-Qaeda reportedly demanded $3 million for his release.
The secret documents note that the kidnapping shows the group's fighters were "running low on funds, and are resorting to drastic measures to get money".
From his captivity, the archbishop managed to send a message to his church, urging them not to pay the ransom because it would be used for "evil" acts. His body was found in a shallow grave a month later.
Today, Al Qaeda does not exist in Iraq as a single entity. Its mantle has been taken on by a disparate range of groups inspired by its ideology, but lacking the resources and popular support that marked out their predecessor.
In truth, the WikiLeaks Iraq war files tell us little that we did not already know about al-Qaeda in Iraq. But they do serve to underline the great irony of the US war in Iraq; that the invasion, billed as part of the so-called "war on terror", did more to inspire al-Qaeda activity in Iraq than it did to undermine it.
It is an irony that has not been lost on former spymaster Manningham-Buller. "Arguably, we gave Osama bin Laden his Iraqi jihad," she said earlier this year.
The thousands of incidents detailed in the leaked documents show just how grateful al-Qaeda was.