The 9/11 attacks on the United States changed the “calculus of risk” posed by Saddam Hussein dramatically, Tony Blair said today.
Opening his long-awaited appearance before the Iraq Inquiry, the former prime minister said that after the devastating attack on the Twin Towers in 2001 they could no longer afford to take the risk that Saddam could reconstitute his illegal weapons programmes.
“Up to September 11 we thought he was a risk but we thought it was worth trying to contain it. Crucially, after September 11 the calculus of risk changed,” he said.
“If September 11 had not happened, our assessment of the risk of allowing Saddam any possibility of him reconstituting his programmes would not have been the same.
“After September 11, our view, the American view, changed and changed dramatically.”
In opening exchanges, inquiry chairman Sir John Chilcot told Mr Blair he would be required to confirm he had told the truth.
Sir John said today's session would focus on three key questions - “Why really did we invade Iraq, why Saddam, and why in March 2003?”
Mr Blair said the 9/11 attacks on the United States completely transformed British policy towards Saddam.
He said: “I would fairly describe our policy up to September 11 as doing our best, hoping for the best but with a different calculus of risk assessment.”
He added: “The point about those acts in New York is that, had they been able to kill more people than the 3,000, they would have.
“My view was you can't take risks with this issue.”
He told the inquiry Iraq had shown “10 years of defiance” and had to be brought “back into compliance”.
But he said the New York atrocities were key to the change in the “assessment” of the security risk.
“That completely changed our assessment of where the risks for security lay,” he said.
“And, just so we get this absolutely clear, this was not an American position - this was my position and the British position.”
Mr Blair insisted that he kept his options for dealing with Iraq open before meeting George Bush at the US president's ranch in Crawford, Texas, in April 2002.
An “options paper” was drawn up in March 2002 which outlined the choices open to Britain for tackling Saddam, including continuing the current containment policy with “smart sanctions”, as well as regime change.
Mr Blair said he consulted foreign secretary Jack Straw, defence secretary Geoff Hoon and Ministry of Defence officials about the courses of action open to him.
He also held a meeting at his country residence, Chequers, before flying to Texas.
Mr Blair said: “We did have a very structured debate with the people. I mean, the fact that it happened at Chequers rather than Downing Street I don't think is particularly relevant to it.
“I think the simple answer is did we consider those other options? - absolutely. That's why we had the paper drawn up.”
Following 9/11, Mr Blair said he wanted to make clear that countries developing weapons of mass destruction “had to stop”.
“The primary consideration for me was to send an absolutely powerful, clear and unremitting message that, after September 11, if you were a regime engaged in WMD, you had to stop,” he said.
Mr Blair said he was aware of the “downside arguments” before sending Britain to war.
“The downside arguments were military action is always something you consider as a last resort,” he said.
He said he was also aware of the possible implications for the relationship with the “Muslim world” and the “Arab world”.
But he added: “My decision was that you cannot afford to have this situation go on.”
Mr Blair sought to play down his comments in a BBC interview with Fern Britton in which he said he would have thought it right to remove Saddam even if he had known that he did not have WMD.
"Even with all my experience in dealing with interviews, it still indicates that I have got something to learn about it," he said.
"I didn't use the words 'regime change' in that interview and I didn't mean in any sense to change the basis.
"Obviously, all I was saying was you cannot describe the nature of the threat in the same way if we knew then what we know now.
"It was in no sense a change of position. The position was that it was the approach of UN resolutions on WMD. That was the case. It was then and it remains."
Mr Blair told the inquiry he believed Saddam was a "monster" before 9/11 but accepted that he would have to make the best of the situation.
He said the American mindset "changed dramatically" after the attacks, adding: "Frankly, mine had as well."
The former prime minister said: "My assessment of risk prior to September 11 was that Saddam was a menace, that he was a threat, he was a monster, but we would have to try and make best."
Shouts from anti-war protesters outside the inquiry venue in Westminster, central London, could be heard inside the building.