It has lasted for 14 months. In public it has heard from 130 witnesses, including two former prime ministers, seven Cabinet ministers, 44 colonels and generals and 15 diplomats.
In private it has taken evidence from countless practitioners of the dark arts of war and diplomacy, from MI6 to GCHQ to the spies who provided intelligence from countries that are normally our allies.
Numerous documents have been published which have shed light on the processes that led to war and the consequences of it. Now there is nothing to do but wait.
Concluding the final public evidence session of his inquiry yesterday, having heard for the third time from the former Foreign Secretary Jack Straw, Sir John Chilcot, the career civil servant who has led the inquiry, said that completing their final report would be a "significant task" which would take months.
"We believe it is important that we do justice to all the oral and the huge amount of written evidence we have received," he said. I don't want to set an artificial deadline on our work at this stage. What I can say is that my colleagues and I wish to finish our report as quickly as possible." So while we wait, what have we learnt?
When Blair decided to go to war
From the first day of the inquiry, witness after witness has made it abundantly clear that the British government had been "planning" for war in Iraq for at least two years before the conflict began.
But the committee was never able to establish that this planning made war inevitable or that Tony Blair had made up his mind to remove Saddam Hussen by force regardless of what took place at the United Nations. What Sir John says here will be an indicator of how hard-hitting his report is overall.
Before Sir John started, "dodgy dossiers", weapons of mass destruction and who was misled by whom was what many people thought the inquiry would focus on. But the intelligence debate has been a sideshow. No witness was foolhardy enough to argue that Saddam did have WMD or direct links to al-Qa'ida. And everyone from Joint Intelligence chairman John Scarlett to the man who commissioned the dodgy dossier, Alastair Campbell, agreed the whole saga was a pretty sorry mess.
The failure in London and Washington to plan better for what happened after Saddam was laid bare by the inquiry. One of the most damaging revelations was that Mr Blair was warned in person that post-war planning in the US was "woefully thin". Major-General Tim Cross, who was sent to Washington to monitor the Bush administration's work in this area, said he told Mr Blair no military action should be taken until a coherent plan had been drawn up to deal with its aftermath. He was ignored. The topic of post-conflict Iraq and its reconstruction is one where Sir John and his committee is expected to pull few punches. Individuals, up to Mr Blair, may come in for stinging criticism.
Was it legal?
This question is of crucial importance to the inquiry – but also one of the most difficult to answer. In his evidence, the former Attorney General Lord Goldsmith acknowledged he had changed his mind on whether a second UN resolution was needed ahead of military action in March 2003 – going from thinking it would be seen as illegal, to being "prepared to accept that a reasonable case" could be made. But what made Lord Goldsmith change his mind? Just yesterday Mr Straw admitted he advised the Cabinet that invasion would be legal without a fresh UN mandate days after Lord Goldsmith said the opposite in private. If Sir John rules the war was illegal then it will be of huge embarrassment to Mr Blair.
Who in government knew what, when and how is another crucial part of the inquiry. If Iraq has come to be seen as "Blair's war" then his style of government was at least partly to blame. Lord Wilson and Lord Turnbull, both heads of the civil service under Mr Blair, told the inquiry that some Cabinet ministers were kept in the dark about the former prime minister's intentions in Iraq. Lord Wilson, the Cabinet Secretary from 1998 to 2002, said Mr Blair assured them in April 2002 that "nothing was imminent". Sir John said at the beginning of his inquiry that it was about learning lessons, not spreading blame, and this is one area where lessons could be easily learnt.
Jack Straw, take three
The only briefing paper given to Cabinet ministers when they had their first big discussion on Iraq in March 2002 was a note for Labour MPs to help them to answer questions from constituents, Jack Straw, the former Foreign Secretary, told the Iraq Inquiry yesterday.
Mr Straw, making his third appearance before Sir John Chilcot, said: "It was a feature of the way that the Prime Minister ran Cabinet that most decisions were made on the basis of oral briefings, having been pre-cooked through... Cabinet committees. The Cabinet was used more for... discussions of that kind rather than acute decisions."