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But six and a half years after then premier Tony Blair led Britain in backing the US-led invasion to oust Saddam Hussein, inquiry chairman John Chilcot stressed that no-one was 'on trial' in the probe, due to last a year.
'What we are committed to, and what I believe the British general public can expect from us, is a guarantee to be thorough, to be impartial, to be objective and fair,' he said in opening remarks.
'As I have said before, we are not a court of law nor are we an inquest nor indeed a statutory inquiry; and our processes will reflect that difference,' he added.
'No-one is on trial here. We cannot determine guilt or innocence. Only a court can do that. But I make a commitment here that once we get to our final report, we will not shy away from making criticisms, either of institutions or processes or individuals, where they are truly warranted.'
One-time top officials from the foreign and defence ministries will outline Britain's policy towards Baghdad in the early 2000s as a five-member committee begins investigating what lessons can be learned from the US-led war.
An appearance by Blair, who took Britain into the conflict, is likely to be the highlight of the inquiry, although he and other Labour government figures are not due to give evidence until next year.
Families of soldiers who died in the conflict said they hoped the inquiry would produce answers.
'We do hope that the committee is going to be honest... I don't know why he died until the end of this inquiry,' said Rose Gentle, whose son Gordon died in Iraq in 2004. She co-founded Military Families Against The War.
She said she would come back to the inquiry when Blair was giving evidence.
'I would ask him why he doesn't meet the families... if mistakes were made,
he's the one that's got to live with it,' she told BBC television.
A small group of demonstrators staged a protest outside the inquiry venue, wearing masks of Blair, former US president George W. Bush and current British premier Gordon Brown, and with fake blood on their hands.
Anti-war campaigners want a ruling on the legality of the conflict, which was carried out without explicit approval by the United Nations Security Council.
Chilcot and his fellow committee members have already met with families of some of the 179 British troops who died during the six-year conflict, who raised issues about whether they were properly equipped and trained.
The inquiry will also be looking into the justification for the war, principally the claim that Saddam had weapons of mass destruction. These weapons were never found.
The first few weeks are intended to establish 'a reliable account' of the main features of British involvement in Iraq between 2001 and 2009, when the war ended with the withdrawal of all but a handful of British troops in July.
Among the first witnesses to be called Tuesday is Peter Ricketts, who chaired the government's top intelligence committee between 2000 and 2001 before taking a senior post at the Foreign Office (FCO) in 2001-03.
There have already been two official probes into elements surrounding the run-up to the invasion, but ministers had refused to hold a full inquiry until after the military deployment had ended.
Ahead of the opening, a report suggested Tuesday that the inquiry was incapable of addressing the key issue of whether the invasion was legal because of a lack of lawyers and judges on its six-member committee.
An unnamed senior judge told The Guardian newspaper that analysing the war's legality was beyond the committee's competence.
