Here are some of the key questions raised:
At what point did he commit British troops to join a US-led invasion of Iraq?
Mr Blair rejected claims that he struck a private deal with President George Bush over Iraq 11 months before the invasion.
But he said he pledged British backing to America if military action was needed to overthrow Saddam Hussein.
He confirmed that, in one-on-one talks at the president's ranch in Crawford, Texas, in April 2002, he told Mr Bush he would be "with him".
Was he always committed to the overthrow of Saddam Hussein, regardless of whether Iraq had weapons of mass destruction?
Mr Blair told Fern Britton in a BBC interview that he would still have thought it right to oust Saddam, even if it had been known that he did not have WMD.
At today's inquiry, he said his comments were "in no sense a change of position" and stressed military action would have been avoided if Saddam had co-operated with the UN.
Was the intelligence about Iraq's supposed WMD doctored in order to make the case for war?
Mr Blair strongly defended his claim in the Government's Iraq dossier, published in September 2002, that the intelligence had established "beyond doubt" that Iraq had WMD.
"What I said in the foreword was that I believed it was beyond doubt. I did believe it and I did believe that it was beyond doubt," he said.
He accepted the dossier should have made clear the now notorious claim that Iraq had WMD which could be launched in 45 minutes referred to battlefield weapons and not long-range missiles.
"It would have been better to have corrected it in the light of the significance it later took on," he said.
Was the attorney general, Lord Goldsmith, pressurised into declaring that military action was legal?
"If Peter (Lord Goldsmith) had said: 'This would not be justified lawfully', we would have been unable to take action," said Mr Blair.
"A lot hung on that decision. Therefore it was important that it was by the attorney general and done in a way which we were satisfied was right and correct."
Mr Blair said Lord Goldsmith - "somebody of extraordinary integrity" - would not have come to his view "unless he believed it".
Did he do enough to prepare for the aftermath of the invasion?
Mr Blair said the Government was "focused" on the aftermath and invested hundreds of millions of pounds in the conflict.
But he accepted there had been a failure to predict the role played by al Qaida and Iran in fomenting the insurgency which broke out.
Did he provide sufficient resources for British forces?
"What became clear in time was not a lack of resources but a lack of security," Mr Blair told the inquiry.
He insisted the military had been properly funded.
"I don't think I refused a request for money or for equipment at any point in the time that I was prime minister," he said.
"And my view, very strongly, is that when you are asking your armed forces to go into this situation, you put everything to one side - other than making sure they have the equipment they need and they have the finances there to back it up."