"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 [weapons of mass destruction], you had to stop."
The inquiry is Britain's third major investigation into the conflict. It was set up last year by Gordon Brown, who took over from Blair as the UK's prime minister, to learn lessons from the conflict.
Alan Fisher, Al Jazeera's correspondent in London, said the inquiry was not going to see "any smoking guns".
"Everyone calls this judgement day, but this isn't really judgement day, it's political theatre. And Tony Blair knows the part he's got to play," he said.
Blair's decision to send 45,000 British troops to Iraq in 2003 was the most controversial move of his 10-year leadership.
Hundreds of protesters, including anti-war campaigners and the families of some of the 179 soldiers who have died in Iraq, have gathered outside the building where Blair is giving evidence.
Many accused the former prime minister on Friday of being a "coward" after it was reported that he had been driven into the building through a side entrance.
Others have called for Blair to be tried for war crimes over his decision to join the US-led invasion.
'Not a trial'
As the hearing opened John Chilcot, the panel's chairman, reminded the audience that the hearing "is not a trial".
Al Jazeera is not responsible for the content of external websites.
Anthony Seldon, a political commentator and biographer of Blair, said the hearing would be a "pivotal day him [Blair], for the British public and for Britain's moral authority in the world".
"This is an enormous day and it goes way beyond him and his own reputation," he said.
But Shane Greer, the executive editor of the UK-based Total Politics magazine, said the inquiry was unlikely to really challenge Blair.
"I don't think we're going to see anything particularly new from this," he told Al Jazeera.
"Tony Blair is a tremendous communicator and this is very much a natural environment for him. He's someone who is used to facing very severe questioning."
The inquiry is likely to focus on the public justification Blair's government gave for the war, notably the so-called dodgy dossier of September 2002.
In a foreword to the dossier Blair wrote that possession of chemical and biological weapons by Saddam Hussein, the then ruler of Iraq, was "beyond doubt" and that he could deploy them within 45 minutes.
But the inquiry has already heard from senior civil servants who said intelligence in the days before the invasion indicated that Saddam's "weapons of mass destruction" had been dismantled.
Blair has always insisted the war was legal, but he has been unable to shake off accusations that intelligence was doctored to support the case for it.
Critics of the inquiry say the five-person team has been too soft on witnesses.