Among the first witnesses called on Tuesday was Peter Ricketts, who chaired the government's senior intelligence committee between 2000 and 2001 before taking a senior post at the Foreign Office (FCO) between 2001 and 2003.
He told the panel that before the war Britain had hoped for a strengthened policy of containment that had been in place since the 1991 Gulf war, reducing the threat posed by Iraq through sanctions, weapons inspections and security measures.
However, Ricketts said some in the Bush administration had a different vision.
"We were conscious that there were other voices in Washington, some of whom were talking about regime change,'' he said, citing an article written by Condoleezza Rice, the then-US national security adviser.
Also making statements at the hearing were William Patey, the former head of the FCO's Middle East department, and Simon Webb, the former head of operational policy at the ministry of defence.
Patey said that Britain had not considering toppling Saddam Hussein.
"In February 2001, we were aware of these drum beats from Washington and internally we discussed it [ousting Saddam],'' he said.
"Our policy was to stay away from that. We didn't think Saddam was a good thing, and it would be great if he went, but we didn't have an explicit policy for trying to get rid of him."
The three witnesses said that the September 11, 2001 attacks on the US were the turning point for attitudes in Washington.
"The shift in thinking was to say that we cannot afford to wait for these threats to materialise. We must be ready to engage potential threats wherever they emerge," Patey said
Chilcot has said that nobody will be on trial in the inquiry, held at a conference centre near parliament in central London, but has also vowed not to shy away from any criticism if the findings warrant it.
"No-one is on trial here. We cannot determine guilt or innocence. Only a court can do that.
"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," he said in opening remarks.
Shane Greer, executive editor of Total Politics, a British political magazine and website, told Al Jazeera that he believes the inquiry will uncover new information about the Iraq war.
"First of all the scope of this inquiry is absolutely unprecedented.
"Already, back in July, Sir John [Chilcot] began speaking with families of injured and killed soldiers ... now he's going onto the spy chiefs, civil servants ... and moving onto politicians.
"So I think we're going to see much more from this inquiry than any previous inquiry, because of course the frame of reference is so much wider, the access to information is so much wider.
"And also the inquiry has been given the power to apportion blame which really is quite incredible."
Chilcot and his fellow committee members have met the 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.
Many families of soldiers who died in the conflict have said they want "honest" answers from the inquiry.
Rose Gentle, whose son Gordon died in Iraq in 2004, said: "We do hope that the committee are going to be honest ... I don't know why he died until the end of this inquiry," she said.
Demonstrators have protested outside the conference venue, with some dressed up as former US and UK leaders with blood on their hands.
Anti-war campaigners are calling for a ruling on the legality of the conflict, which was carried out without explicit approval by the United Nations Security Council.
Two official investigations into the run-up to the war have already taken place, but ministers had refused to hold a full inquiry until after the military deployment had ended.
Analysts have said the inquiry is 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.