So why did George Bush decide to invade Iraq? Nearly four years and hundreds of thousands of casualties later, the reasons appear both as obvious and as elusive as they were in the spring of 2003.
The official reasoning was always straightforward. Key among the claims included in the so-called Iraq War Resolution passed by Congress in October 2002 was that Iraq "poses a continuing threat to the national security of the United States and international peace and security in the Persian Gulf region". It added that Saddam's regime harboured chemical and biological weapons and was seeking to develop a nuclear arsenal.
In an address to the nation just three days before the invasion, Mr Bush declared: "Intelligence gathered by this and other governments leaves no doubt that the Iraq regime continues to possess and conceal some of the most lethal weapons ever devised."
It quickly became clear that central claim was not true, and it became equally clear the administration had been manipulating uncertain and "caveated" intelligence to make the case for a war that had been decided on long before. The famous Downing Street memo suggests that as early as July 2002 "intelligence and facts were being fixed around the policy". Indeed, within hours of the attacks of 9/11, senior elements within the administration were seeking for a strike against Iraq even though there was no evidence it was involved.
But if the alleged threat of WMD was based on manipulated intelligence – some provided by Iraqi exiles such as Ahmed Chalabi's Iraqi National Congress - what else motivated the US? Many remain convinced the overwhelming factor was a desire to control Iraq's oil supplies, the second largest proven reserves in the world. Such a view has been reinforced by recent recommendations of Iraq Study Group which said: "The United States should assist Iraqi leaders to reorganise the national oil industry as a commercial enterprise, in order to enhance efficiency, transparency, and accountability."
Veteran dissident Noam Chomsky said: "It is glaringly obvious that Iraq is estimated to have the second largest energy reserves in the world and is right at the heart of the world's major energy producing region, and that establishing a client state in Iraq would considerably enhance policies that go back to the dawn of the oil age, and in particular to the post-war period when the US was taking over global domination, and established as a very high and natural policy principle the need to control this ‘stupendous source of strategic power'."
He added: "It takes remarkable obedience to authority to believe that the US would have 'liberated' Iraq - or taken revenge - if its main exports were lettuce and pickles, and the major petroleum resources were in the South Pacific."
Some point out that a desire among some in government to oust Saddam predated 9/11, and suggest in the aftermath of those attacks, a climate existed in which it was easier to pursue an invasion. Indeed, among the signatories to the 1998 letter from the neo-con Project for the New American Century calling on President Clinton to take on Saddam were former Defence Secretary Donald Rumsfeld and his deputy, Paul Wolfowitz.
Mr Wolfowitz later said Saddam's alleged possession of WMD was just one of many reasons for invading. "For bureaucratic reasons, we settled on one issue, weapons of mass destruction, because it was the one reason everyone could agree on," he said.
David Swanson, a founder of afterdowningstreet.org, a coalition of peace and activist groups, said: "The one thing we know is that the reasons they told us were false. [I think] they wanted an Iraq that looked free but isn't and they wanted to control it¿They wanted the oil and the power that comes with controlling that oil and making profits for British and US oil companies."
Did other factors influence Mr Bush? Was he seeking revenge against "the guy who tried to kill my dad" – a reference to an alleged plot to kill the president's father during a visit to Kuwait in 1993 or was there even a broader strategic rationale, one that would benefit Israel – something claimed by peace activist Cindy Sheehan.
What does seem certain is that there was a confluence of factors and interests coming together in the aftermath of 9/11 that allowed Mr Bush to proceed to war with little opposition from the Congress, or indeed, the media.