Tony Blair has voiced "sorrow, regret and apology" for the "failures" in Iraq, but insisted he stood by his decision to go to war.
Following the publication of the Chilcot Report, the former prime minister acknowledged that some of the families of the 179 British personnel killed in the conflict could "never forget or forgive" him for what happened.
But at an emotional news conference, he said he firmly believed he had done the "right thing" and that the world was a "better place" without Saddam Hussein.
He said the decision to remove Saddam was the "hardest, most momentous, most agonising" of his 10 years in office.
While Mr Blair accepted the report contained "serious criticisms", he said it showed British Parliament was not misled, there was no secret commitment to war, intelligence was not falsified and the decision was made in "good faith".
"A decision had to be taken and it was for me to take as prime minister. I took it, I accept full responsibility for it, I stand by it," he said, his voice close to breaking.
"I only ask with humility that the British people accept that I took this decision because I believed that it was the right thing to do based on the information that I had and the threat I perceived and that my duty as prime minister at that moment in time was to do what I thought was right.
"At moments of crisis such as this it is the profound obligation of the person leading the government of our country to take responsibility and decide.
"Not to hide behind politics, expediency or even emotion but to recognise that it is the privilege above all others to lead this nation.
"But the accompaniment of that privilege when the interests of our nation are so supremely and plainly at stake is to lead and not to shy away, to decide and not to avoid decision, to discharge that responsibility and not to duck it."
Mr Blair acknowledged that there had been failures of "planning and process" in the course of the invasion and its aftermath.
"The intelligence assessments made at the time of going to war turned out to be wrong, the aftermath turned out to be more hostile, protracted and bloody than we ever imagined. The coalition planned for one set of ground facts and encountered another," he said.
"And a nation whose people we wanted to set free and secure from the evil of Saddam became instead victim of sectarian terrorism.
"For all of this I express more sorrow, regret and apology than you may ever know or can believe."
He said he "profoundly disagreed" with claims that Saddam's removal had caused the upsurge in terrorism in the Middle East and that it would have been better to have left him in power.
"Saddam was himself a wellspring of terror, a continuing threat to peace and to his own people," he said. "The world was - and is - in my judgment a better place without Saddam Hussein."
Mr Blair insisted he did not commit Britain to war months before the invasion began with his note to US president George Bush which declared "I will be with you, whatever".
"There was no rush to war," he said.
Mr Blair maintained that Saddam needed to be toppled in order to stop weapons of mass destruction (WMD) falling into the hands of terrorists who could perpetrate a "British 9/11".
Mr Blair said he had apologised over more than half a decade for shortcomings in the intelligence which formed the basis for a 2002 dossier in which the government set out its case on Saddam's supposed WMD capabilities.
"The decisions I made I have carried with me for 13 years and will do so for the rest of my days," he said.
"There will not be a day of my life where I do not relive and rethink what happened.
"People sometimes ask me why I spend so much time in the Middle East today - this is why."
He accepted the UK should have pressed the US harder to make proper plans for the aftermath of the invasion. But he noted that the inquiry had been unable to identify "alternative approaches which would have guaranteed greater success".
Mr Blair claimed Saddam would have been emboldened had the threat of military action been withdrawn and would probably have carried on for years to come.
That could have led to a brutal repression of the Iraqi people during and after the Arab Spring uprisings of 2011, replicating the horror seen in Syria today, he insisted.
"Saddam, in my view, was going to pose a threat for as long as he was in power," he said.
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Mr Blair insisted his comments about being with America "whatever" referred to a determination to withstand the political blow-back.
"I was going to be with America dealing with this whatever the political difficulties, whatever problems there were going to be. I was going to put us alongside America dealing with this, but it had to be done right. I persuaded President Bush to go down the UN route - that was the vital thing we were doing."
Mr Blair insisted he gave orders that the military was to be given all the resources it needed for adequate equipment in Iraq and Afghanistan.
"Both myself and Gordon Brown at the time made it absolutely clear that whatever was requested should be given. I think we were absolutely prepared for the campaign to remove Saddam because it was brilliantly successful."
Asked what he felt about threatened legal action from families of the dead, Mr Blair said: "I understand their - not just their grief - but I understand their anger and their concern, but I need people also to try to put themselves back in my shoes at that time taking that decision, and understand why I took that decision, why I think it was right, and why I will never, ever accept that those troops who got injured or gave their lives in Iraq did so in vain."
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Asked how he would respond to demands by families of the war dead to look them in the eye and deny he misled the country, Mr Blair said: "I can look not just at the families of this country, but the nation in the eye, and say I did not mislead this country. I made the decision in good faith on the information I had at the time.
"What I cannot do, and will not do, is say I believe we took the wrong decision. I believe I made the right decision, and that the world is better and safer as a result of it.
"As this report makes clear - and it does when you go through the report - there were no lies, there was no deceit, there was no deception, but there was a decision, and it was a controversial decision, a decision to remove Saddam, and a decision to be with America."
Mr Blair said he was apologising for the mistakes in the planning and process of the war, but he stood by the key decision to invade.
"If I was back in the same place with the same information, I would take the same decision, obviously, that's the decision that I believe was right," he said.
Mr Blair insisted many more people had died in Syria than Iraq "when we didn't intervene and remove the dictator".
The ex-prime minister added that the roots of the terrorism which has erupted since the invasion "go so much deeper than Iraq".
Mr Blair insisted he had never lied, and did not mislead parliament, stating: "Please stop saying I was lying, or I had some kind of dishonest or underhand motive."