On 25 February 2003, less than a month before the invasion of Iraq, and in one of his most important speeches as prime minister, Tony Blair stood up before the House of Commons to deliver a statement on Saddam Hussein and the crisis in the Middle East.

“I detest his regime," he said, in a passionate address to sceptical MPs on both sides of the house. "But even now, he can save it by complying with the UN's demand. Even now, we are prepared to go the extra step to achieve disarmament peacefully." He added, solemnly: "I do not want war."

It is perhaps on this single Commons statement that the entire case against Blair rests. Is it true that he did "not want war"? Could Saddam have saved his regime? As the Conservative former prime minister John Major - who supported the war - remarked in a BBC radio interview in January: "The suspicion arises that this was more about regime change than it was about weapons of mass destruction."

“Regime change" is a euphemism for the unilateral and often violent overthrow of a ­foreign government. Regime change is illegal under international law - and has become, in recent days and weeks, the chief focus of the Iraq inquiry led by the former Whitehall mandarin John Chilcot.

Black and white

Since the inquiry began on 24 November 2009, serving and former officials from the Foreign Office, the Ministry of Defence, Downing Street, the Joint Intelligence Committee, MI6 and the armed forces have given evidence in front of Chilcot and his colleagues at the Queen Elizabeth II Conference Centre in London. But have they added anything significant to what we already knew? Over the years, numerous revelations - including leaked official memos and minutes - have suggested that Blair, in spite of his repeated denials, signed up not simply to disarmament but to regime change in Iraq a full year before the invasion in March 2003.

“We spent a long time at dinner on Iraq. It is clear that Bush is grateful for your support and has registered that you are getting flak," wrote David Manning, Blair's foreign policy adviser, in a secret memo to the prime minister after dining with President Bush's national security adviser Condoleezza Rice on 14 March 2002. "I said that you would not budge in your support for regime change but you had to manage a press, a parliament and a public opinion that was very different than anything in the States."

Blair's decision "not to budge" in support of regime change was confirmed by a subsequent memo, this time from the then British ambassador to Washington, Christopher Meyer, to Manning. This summarised a conversation he had had with Paul Wolfowitz, the US deputy defence secretary, on 17 March 2002: "I opened by sticking very closely to the script that you used with Condi Rice. We backed regime change, but the plan had to be clever and failure was not an option."

By the time Blair went to discuss the issue of Iraq with Bush in Crawford, Texas, on 6 April 2002, regime change - and not just disarmament - seems to have become his settled, private position. Alastair Campbell's diary entry for 2 April 2002 notes that participants at one meeting "discussed whether the central aim was WMD or regime change", and that "TB felt it was regime change".

Asked at the Iraq inquiry on 30 November 2009 whether the Blair government had secretly committed to the US policy of "regime change", however, Manning replied with a Clintonesque flourish of obfuscation and redefinition: "It depends what you mean, I think, by 'regime change', because . . . if Saddam Hussein accepted the provisions of, as it turned out to be, UN Security Council Resolution 1441, the situation on the ground in Iraq would be so profoundly different that the regime would have changed itself."

One well-respected international lawyer, who asked not to be named, described Manning's tortuous position as "hopeless . . . complete crap".

In the run-up to the 2005 general election, Blair appeared on a television programme that I produced and, when confronted with the Manning memo by the interviewer, Jonathan Dimbleby, claimed that regime change had been a method of last-resort disarmament: "If you couldn't enforce the UN resolutions by any other route, then you'd have to go down the route of regime change."

Blair has continued to deny that he signed up to a US policy of regime change in Iraq long before the start of the war, despite evidence to the contrary uncovered in the various leaks and memos. In a television interview shortly after leaving office, in November 2007, he said: "It is complete rubbish that, when I went to see President Bush, I said, 'Right, OK, I'm up for it.' "

However, those who had discussions with him in the run-up to the invasion in early 2003 have their doubts. In a little-reported meeting at Downing Street with six of Britain's leading academic experts on Iraq in November 2002, Blair is said to have paid little attention to the logistical difficulties of invading or occupying Iraq, instead exclaiming: "But he [Saddam] is evil, isn't he?" Black and white. It was that simple. Blair seemed unable to engage with the complexities of Iraq, and eager to dismiss the doubts of the experts. "For Blair, Saddam was evil," said Hans Blix, the former chief UN weapons inspector, when I spoke to him.

Witch-hunters

The debate over the Iraq war has often revolved around whether Blair misrepresented (or "sexed up") the intelligence on WMDs. A former Blair adviser defends him, saying: "There is a little bit of rhetorical exaggeration in what Tony said at the time, though he always believed that there were WMDs in Iraq, as did I, so it was exaggeration rather than lying."

For Blix, however, the "exaggeration" was inexcusable. "Blair was in a position to ask the people in intelligence: 'What is your evidence?'" he argues. He compares Blair and Bush to latter-day "witch-hunters": they were only interested in evidence that suggested the Iraqis possessed WMDs and were thus guided by erroneous hunches and misguided assumptions. "It is one thing to act on your hunches, in a less serious context; but to go to war on a hunch, and on the basis of fairly unsubstantiated evidence, is quite another," Blix says now.

Enforcing disarmament ultimately became the casus belli for the Iraq war. "'WMD' was indispensable for them [Bush and Blair]," Blix told me. "That's why they settled for it."

Rodric Braithwaite, a former chairman of the Joint Intelligence Committee, told me that the inquiry has not "dealt with what happened between January and March 2003", and nor have the media. It had become clear by January and February 2003 that much of the purported intelligence that the UK had disseminated to the public, including a substantial portion of the material in the notorious September dossier (the intelligence document the British government published on 24 September 2002 to bolster support for military action against Iraq) was incorrect. But, says Braithwaite: "He built up the argument after September 2002, not down. That's what I call warmongering."

Indeed, if Blair was genuinely interested in the issue of weapons of mass destruction and disarmament, rather than regime change for its own sake, why didn't he order a reassessment of intelligence on Iraq's WMDs after the introduction of UN weapons inspectors inside the country and their failure to discover any banned weapons, between November 2002 and March 2003?

“I think this was one of the most significant things of the whole story," says Blix. "We had time to go to about three dozen of these sites and in no case did we find any weapons of mass destruction.

“One thing that is often forgotten is that the responsibility for starting belligerent action should be judged by the time that action was taken. Blair knew a lot more by mid-February and early March. It's against . . . what he knew then that he should be judged."

So will any inquiry ever establish what many of his disparagers have long believed - that Blair "lied" us into an illegal war? "It doesn't make sense to call him a liar," says Braithwaite. "I think he convinced himself. Blair's justification is that he believed it at the time. But you don't pay your prime minister to believe something. You pay him to get his judgements right."

Home front

It is often forgotten that Blair, on more than one occasion, attempted to make connections between Saddam Hussein, Osama Bin Laden and al-Qaeda. Many assume that the suggestion of links between Bin Laden and Saddam was made by the Bush administration. But on 21 January 2003, Blair told the liaison committee of MPs: "There is some intelligence evidence about loose links between al-Qaeda and various people in Iraq . . . It would not be correct to say there is no evidence whatever of linkages between al-Qaeda and Iraq."

On 29 January 2003, a Foreign Office spokesman went further, saying: "We believe that there have been, and still are, some al-Qaeda operatives in parts of Iraq controlled by Baghdad. It is hard to imagine that they are there without the knowledge and acquiescence of the Iraqi government."

Since the 2003 invasion, Iraq has become a haven for al-Qaeda terrorists. Toppling the regime in Baghdad has emboldened Bin Laden and his client operations. But were there pre-war links between Saddam Hussein and the al-Qaeda leadership?

“The terrorism experts and CIA were pretty convinced at the time that there were none," says Marc Sageman, a former CIA case officer who is one of the world's leading experts on Islamist terrorism. "Al-Qaeda hated Saddam's regime."

A classified document, written by defence intelligence staff in early January 2003, flatly contradicted the Downing Street and Foreign Office claims made later that same month. It noted how Bin Laden viewed Iraq's ruling Ba'ath party as anti-Islamic. To him, it was an "apostate regime". "His aims are in ideological conflict with present-day Iraq," the document concluded.

“There was no link whatsoever between Bin Laden and Saddam," says Sageman. "It was all spurious." Indeed, the focus on "spurious" links between al-Qaeda and "people in Iraq", Sageman tells me, exposed a "lack of integrity" on the part of the US and UK governments.

This lack of integrity extends to the nature and origins of the post-Iraq terror threat. On 22 January this year, the Home Secretary, Alan Johnson, announced that the Joint Terrorism Analysis Centre had raised Britain's international terrorism threat level from "substantial" to "severe" - its second-highest level of alert.

Yet those who prosecuted the war have consistently refused to acknowledge or accept any link between the invasion of Iraq and terrorist attacks on the streets of London and beyond.

As early as February 2003, the Joint Intelligence Committee reported that al-Qaeda and associated groups continued to represent "by far the greatest terrorist threat to western interests, and that that threat would be heightened by military action against Iraq". In July 2009, Eliza Manningham-Buller, who was head of MI5 in the run-up to the invasion, revealed that she warned ministers that war with Iraq would increase the terrorist threat to the UK. “I said it as explicitly as I could. I said something like, 'The threat to us would increase because of Iraq.'"

In his unseemly rush to war and regime change, Blair shared none of this with parliament or the public. Of course, Britain may have been in the terrorists' cross hairs from the moment Tony Blair pledged to stand "shoulder to shoulder" with President Bush and the United States in the aftermath of the 11 September 2001 attacks. But the so-called blowback against British interests began only after the Iraq invasion.

Eight months later, on 20 November 2003, al-Qaeda suicide bombers carried out attacks against the British consulate and the HSBC building in Istanbul, Turkey, killing 27 people (including three British citizens). Then came the 7 July 2005 terror attacks in London, which killed 56 people.

For Sageman, author of the recent book Leaderless Jihad, the invasion of Iraq was a "recruiting sergeant" for Osama Bin Laden and his terrorist allies. "You see very much from the transcripts of the Operation Crevice trial [relating to the thwarted 2006 fertiliser bomb plot to blow up the Bluewater shopping centre in Kent and other public buildings] that the plotters were discussing the invasion of Iraq," he tells me. "Iraq is the moment when British jihadists started focusing on attacks inside the UK, because the British government had invaded an Arab country."

The Crawley-born ringleader of the fertiliser bomb plotters, Omar Khyam, confirmed that Britain was not a terror target until the 2003 Iraq invasion, saying at his trial: "People didn't view the UK the same as the US. The Iraq war was the final straw. Whereas before, myself and others made excuses, now they believe the UK and America needed to be attacked."

Speaking in a pronounced Yorkshire accent in his suicide video, Mohammad Sidique Khan, the leader of the 2005 London bombers, cited Britain's role in the invasion of Iraq as justification for his murderous actions: "Until we feel secure, you will be our targets. Until you stop the bombing, gassing, imprisonment and torture . . . we will not stop this fight."

Only three months before the London bombings, in April 2005, the Joint Intelligence Committee concluded that "the conflict in Iraq has exacerbated the threat from international terrorism and will continue to have an impact in the long term. It has reinforced the determination of terrorists who were already committed to attacking the west and motivated others who were not."

Ironically, and tragically, the least credible reason advanced for going to war - terrorism - has been its most catastrophic consequence for the UK.

Crime of aggression

Did Blair break the law in ordering the invasion of Iraq, without a second UN resolution, and in pursuit of regime change rather than disarmament? Once again, the evidence in the public domain, even before the start of the Chilcot inquiry, suggests that he did.

Much of the controversy concerns the then attorney general, Peter Goldsmith, who had warned Blair in July 2002 that a war to remove Saddam from power would be a serious breach of international law as well as the UN Charter. Goldsmith continued to doubt the legality of military action until very nearly the eve of war a year later. His qualified assessments of the arguments, and his own doubts about them, were detailed in a memo to Blair on 7 March 2003. But ten days later, on 17 March, he issued a much shorter and definitive version of his legal advice, declaring an invasion legal. His critics, including Elizabeth Wilmshurst - the deputy legal adviser at the Foreign Office who resigned in March 2003 over Iraq, referring to it as a "crime of aggression" - have accused Goldsmith of changing his mind under pressure from the then prime minister and his aides.

“The absolutely key thing is the legal advice. People can disagree about whether or not to go to war, but the question as to whether it was legal is a really big one," says one former Downing Street adviser. Another ex-aide, a Blair loyalist, tells me that his former boss is "vulnerable" on the issue of legality and refers to the "whole Goldsmith business" as being "very murky".

For Hans Blix, there is nothing murky whatsoever. Goldsmith, he says, "was acting like a defence attorney. Blair could not bail out [of the invasion] and so Goldsmith helped him justify it." Blix, like his former boss at the United Nations, Kofi Annan, considers Iraq to be an "illegal war".

In fact, so do most international lawyers. Philippe Sands QC, a professor of international law and director of the Centre on International Courts and Tribunals at University College London, says he believes, unequivocally, that the 2003 invasion was illegal under international law. "In the UK, beyond those associated with the government's effort, I cannot think of a single international lawyer who thinks the war was lawful," he says. "Not a single name comes to mind. That's got to be telling."

The New Statesman has learned from a senior legal source that not one member of Britain's new 12-justice Supreme Court believes that the war was lawful. One former law lord, Johan Steyn, has called on the Iraq inquiry to publish an interim report before the general election declaring the war illegal.

In January, for the first time, a state that supported the war, the Netherlands, held an independent inquiry that unanimously and unambiguously concluded that the invasion violated international law and lacked a proper "mandate". Will the Chilcot inquiry follow the example of the Dutch? Sands is sceptical: "They blew it by not having a single lawyer on the panel. The members of the panel are not in a position to express an authoritative view on that subject."

Nor does he think that Blair will ever face a domestic criminal investigation or an international tribunal - despite a recent poll for the Sunday Times suggesting that a quarter of the public want to see him tried for war crimes. But, says Sands: "The possibility of a national prosecutor going after Blair in some foreign jurisdiction is not high but cannot be excluded. He may have to modify his travel arrangements."

Final verdict

So what of Tony Blair's legacy, in the wake of the Iraq war and Chilcot? Will he be held to account for going to war without a second resolution, or for misrepresenting the threat of weapons of mass destruction or the link between al-Qaeda and Iraq?

Given the nature, history and composition of such inquiries, it is unlikely. “These people are not going to end up saying that Blair took the country into an illegal war and should go to The Hague," says Braithwaite. "It's not going to end in the way that people who would like to see him strung up would like."

However, Braithwaite argues that Blair "has lost all political credibility": "He sticks to the American right and media." After Blair, his successor, Gordon Brown, will be testifying before Chilcot. Seven years on from the war, Blair and Brown are, in effect, on (televised) trial. The Tories are poised to benefit from this, despite their own misguided support for the Iraq invasion.

It is often forgotten that the Conservative leadership went further than Blair in exaggerating a direct Iraqi threat to Britain. In September 2002, Iain Duncan Smith, the then leader of the Tory party, claimed that Iraq was developing ballistic missiles that would have "the capacity to strike most of Europe, including London".

David Cameron, too, was a supporter of the war. "I look back over what I said to my constituents and the argument about weapons of mass destruction was just one of the points that I put," he said in a recent interview, echoing Blair's line that there was a separate case for regime change. "I think the fact that Saddam Hussein was in breach of so many UN resolutions, and was such a menace to the region, were also relevant points."

The Conservatives may also be culpable, but oppositions do not launch wars. And so, Iraq will always be Blair's war. He understands this, and is unrepentant. "I'm afraid in one sense it's worse than people think . . . I believed in it. I believed in it then, [and] I believe in it now," he said, after leaving office in 2007.

And he continues to believe that he will be judged in a higher place: "I'm ready to meet my Maker and answer for those who have died as a result of my decisions."

Mehdi Hasan is senior editor (politics) of the New Statesman.

An earlier version of this article carried different wording for comments by Philippe Sands, QC. These comments have been amended.