The report - which is more than 2.5 million words long - is being released seven years after the inquiry began
In the UK, John Chilcot has given the first hint that his report into the Iraq War will level criticisms at the politicians and military leaders who took Britain to war.
The Iraq Inquiry, which was led by Mr Chilcot, will today publish its long-awaited report, seven years after the inquiry began and 13 years after British troops crossed into Iraq.
Speaking ahead of the publication, Mr Chilcot said: "The main expectation I have is that it will not be possible in future to engage in a military or diplomatic endeavour on such a scale and of such gravity, without really careful challenge, analysis and assessment.
He added: "I made very clear right at the start of the inquiry that if we came across decisions or behaviour which deserved criticism then we wouldn't shy away from making it.
"And indeed, there have been more than a few instances where we are bound to do that. But we shall do it on a base of a rigorous analysis of the evidence that supports that finding."
Key political and military figures, including former prime minister Tony Blair, are expected to be criticised in the report.
They are likely to be under fire for the way intelligence about Iraq's alleged weapons of mass destruction was used, and failures to plan for a post-Saddam Hussein Iraq.
The relatives of the 179 men and women who died in Iraq will get advance sight of the report, which runs to 2.6 million words.
Some say if Mr Blair is found to have deceived people about the reasons for going to war they will push for criminal action against him.
Rose Gentle's son Gordon was just 19 when he was killed by a roadside bomb while travelling in a lightly armoured Snatch Land Rover in Basra in 2004.
She says the vehicle was not fit for purpose and the report is likely to identify issues with equipment, resources and point to military failures after the invasion.
"I need the truth. I really just need the truth as to why," she said. "I think that's a question that's in your head at night, in the morning.
"You're asking why? Why was he there? Why was he killed there? Why did we go into Iraq?"
From what she has heard in the years since Gordon died, Mrs Gentle is convinced Mr Blair made a secret pact with George Bush to take on Hussein by force.
The report will make public never-seen-before correspondence between the UK and US leaders in the run-up to the war.
Key questions which may be addressed include:
Christopher Meyer, former UK ambassador to the US, was in Crawford at the time and remembers Mr Blair and Mr Bush having private meetings.
"I think what Bush would have heard from Blair was something like: 'Well, George, whatever you decide to do, you know you can count on my support'," he said.
But he added: "If they agreed to go to war at Crawford ... it would mean Bush and Blair had for nine months lied to their respective publics.
"I simply don't believe that happened."
But the families of those who died will be looking to the inquiry's chairman to decide if there is evidence Mr Blair misled Parliament and the public.
Mr Blair told MPs in a strongly worded dossier in September 2002 there was extensive and detailed evidence Saddam Hussein had WMDs and could deploy them in 45 minutes.
Both claims turned out to be wrong and months of searching by UN weapons inspectors turned up nothing.
Hans Blix, who led that search, said: "Well, politicians spin - they want to convince the public and they go too far.
"When you're trying to convince someone to go to war you have to have more responsibility."
Mr Blix believes the war was illegal and the report will look into claims that former attorney general Lord Goldsmith was persuaded to change his advice at the 11th hour, giving backing for the war without a second UN resolution.
Civil servants, intelligence chiefs and senior members of the military are expected to face some censure for mistakes made in the run-up to the war, its execution and aftermath.
Security in southern Iraq, which the British controlled, deteriorated fast.
Mr Blair, who has apologised for failures in post-war planning - if not for getting rid of Saddam Hussein - is expected to give his response after publication.