The official inquiry into Britain’s involvement in the invasion of Iraq in 2003 — described as a “war of choice” rather than a war of necessity — should expose for the first time the political and military disagreements that led to the decision by Tony Blair to commit 45,000 troops to support the American campaign against Saddam Hussein.

Two inquiries have already been held — the Hutton inquiry into the circumstances behind the suicide of David Kelly, the Ministry of Defence expert on Iraq’s weapons of mass destruction programme, and the Butler inquiry into the intelligence used to justify the war.

Neither inquiry, however, addressed the key political decision-making process, including the individual views of Cabinet ministers, the extent of the planning for the aftermath of the invasion, the discussions with the Americans, the scale of the concerns expressed by military chiefs over the legality of the war and the level of preparation for the war, including the options considered in the event of the campaign going wrong.

On the evidence that has emerged during the last six years of Britain’s campaign in Iraq, codenamed Operation Telic, a number of key questions remain to be answered:

1. At what stage did the Blair Government decide that removing Iraq’s supposed weapons of mass destruction (WMD) was no longer the only objective and that regime-change was the real aim?

Ministers and officials must have suspected that with such thin intelligence available on the scale and whereabouts of the weapons, there would have to be other reasons for invading Iraq. Mr Blair and Mr Bush agreed that the purpose behind the invasion was to “disarm Iraq of weapons of mass destruction, to end Saddam Hussein’s support for terrorism and to free the Iraqi people”.

However, the British Government — and, importantly, MI6 — never supported Mr Bush’s claim that Saddam was linked to al-Qaeda and thus, indirectly, to 9/11. Regime-change was never put forward officially as a campaign objective.

Indeed, Jack Straw, then Foreign Secretary, declared in public that regime change was not the motivation for going to war with Iraq and insisted that, provided the WMD was found and destroyed, there was no reason why Saddam Hussein should not remain as leader in Baghdad.

However, this was not the view in Washington and Mr Blair would have known from an early stage in his discussions with President George Bush and Vice-President Dick Cheney that toppling Saddam was the main American objective. There was never any question, in Mr Bush’s mind, of seeing Saddam still in power after he had assembled 100,000 US troops in Kuwait for the invasion. Was this a reality fully understood by the UK War Cabinet?

2. What was going on behind the scenes as the demand for a "second UN resolution" was abandoned?

The British position was that the United Nations Security Council needed to approve a mandate for going to war with Iraq, indicating that the existing UN resolutions — 17 of them — and in particular Resolution 1441, passed in 2002, was not sufficiently strong to justify military action. Mr Blair persuaded Mr Bush to seek what was called a “second resolution”. In early 2003, the US, Britain and Spain proposed a resolution which sought a mandate for the use of force to remove the suspected WMD.

When it became clear that this move was not going to succeed — France would have vetoed it — the resolution was withdrawn but plans for the invasion went ahead on the basis of Resolution 1441, as well as the persistent failure of Saddam to dismantle his weapons programme, as demanded by the UN Security Council on many previous occasions.

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