The Chilcot Inquiry will look into Britain's stewardship of Basra and southern Iraq following the 2003 invasion as it takes evidence from two defence secretaries who held office after the toppling of Saddam Hussein.
Des Browne and John Hutton are likely to be asked about the UK's handling of the insurgency in southern Iraq, which claimed the lives of dozens of British personnel.
And they will be challenged over claims that control of Basra was effectively ceded to extremist militias until the 2008 Charge of the Knights offensive by the Iraqi army, backed by US forces.
Mr Browne and Mr Hutton were relatively junior ministers in Tony Blair's administration at the time of the invasion of Iraq, and while they backed the war they did not play a major part in the decision to commit UK troops.
Their involvement in Iraq came when Mr Browne served as Defence Secretary between 2006 and 2008, to be followed by Mr Hutton, who was in charge when the last UK combat troops were withdrawn from the country last year.
Today's evidence session will be a relatively low-key event ahead of the potentially explosive appearances later this week of former attorney general Lord Goldsmith and Mr Blair himself.
The inquiry tomorrow hears from the Foreign Office's most senior legal advisers in the run-up to war, who are expected to tell chairman Sir John Chilcot they believed invasion was unlawful without a second United Nations resolution.
On Wednesday, Lord Goldsmith is likely to be asked about claims that he changed his mind on the legality of war in the weeks before hostilities commenced.
And Friday will witness the most charged moments of the inquiry so far, as Mr Blair undergoes six hours of questioning about his secret discussions with US President George Bush, his involvement in the preparation of controversial dossiers on the threat from Iraq and his decision to commit British troops.