Inquiries have looked at different aspects of the conflict
The UK public inquiry into the Iraq war, which has just begun, follows a series of investigations by the US into the 2003 invasion.
In September 2006 the US Senate Intelligence Committee published one of the definitive public accounts of the intelligence used to justify the Iraq war.
Its 400-page report, three years in the making, laid bare the justifications for the invasion - and found little or no evidence to back a raft of claims made by the US intelligence community concerning Saddam Hussein's weapons of mass destruction [WMD].
The report came just weeks before George Bush's Republicans were trounced in mid-term elections dominated by the issue of the war.
Days after the defeat, then Defence Secretary Donald Rumsfeld, one of the war's chief architects, quit the government.
Those weeks may have been a significant moment in the reckoning of the Iraq war in the US, but the intelligence committee's report was just one of a number of inquisitions into all aspects of the conflict that have been going on since the 2003 invasion.
Eighteen months before the Senate report, the Silberman-Robb commission - set up by President Bush in early 2004 - had reported in no uncertain terms that US intelligence had been "dead wrong" in judging that Iraq had been developing WMD before the invasion.
Led by retired judge Laurence Silberman, the report was highly critical of intelligence failings, but it had attracted criticism from Mr Bush's opponents who had wanted it to report back before the November 2004 presidential election.
While it was not conceived as a backward-looking inquiry into the Iraq war, one of the most high-profile investigations of the conflict was the Baker-Hamilton Iraq Study Group.
Set up by Congress in March 2006, the 10-member panel was tasked with assessing the situation on the ground in Iraq and coming up with recommendations.
It met or spoke to more than 170 individuals, including Iraq's leaders, President Bush, UK Prime Minister Tony Blair, ambassadors and other senior officials, analysts and media representatives from among Iraq's neighbours and the US.
In December 2006 it concluded the situation in Iraq was "grave and deteriorating" - strongly urging a pull back of US forces once the situation allowed.
Meanwhile, within the US Congress itself, hundreds of witnesses have testified at scores of hearings on Iraq in recent years.
High profile testimony
Dozens of select committees in the US Congress routinely probe a wide range of government activities; seeking opinions on proposed legislation, overseeing the activities of a department, the implementation of laws or even simply providing testimony on topics of current interest.
In 2003/2004 for instance, the Congressional year in which the war began, dozens of hearings dealt with subjects ranging from "the complex task of coordinating contracts amid the chaos and the rebuilding of Iraq", to "next steps: What will an Iraq five year plan look like?".
Regular Congressional hearings on Iraq have heard from top US officials
In some of the highest profile testimony, senior US government officials including then Deputy Defence Secretary Paul Wolfowitz and Joint Chiefs chairman Gen Richard Myers faced hostile public questioning on the reasons for and conduct of the war.
In another high profile hearing in June 2005, the senate armed services committee heard from Mr Rumsfeld, who defended US policy in Iraq, while then US Gulf commander General John Abizaid admitted that a rising number of foreign fighters were entering the country.
Witnesses at Congressional hearings range from the highest officers of state to ordinary members of the public, who may have specific connections to a particular issue.
Most give evidence in public - or submit written statements - while some give secret testimony where national security is at stake.
Most people give evidence - and contribute to public policy debate - willingly, but subpoenas are issued infrequently, mostly in the course of investigative hearings.
While British Prime Minister Tony Blair is expected to give public evidence to the UK's Iraq war inquiry, there has been no such effort to draw public testimony from President Bush in the United States, where the separation of powers between the executive office of president and the legislature is enshrined in the Constitution.