There seem to me to be three main things that we need to inquire into:
1. The process by which the decision was made to go to war in Iraq. This needs to include the questions about the intelligence and the use of intelligence on WMDs, questions about when Blair decided to press for an invasion - and how that interacted with Bush's decision, because some of the information that has come out suggests that Blair had reached his decision independently even before Bush was firmly committed to war.
In looking at intelligence, we need to be going back to the 80s when Saddam used chemical weapons on the Kurds and the Iranians, and to examine how intelligence looked at the disarmament process; UN inspectors and so on, because I think it's now clear, with hindsight, that the inspectors were more effective than even they believed they were at the time, and that intelligence underestimated the degradation of the chemical and biological arsenalsand completely missed the fact that Saddam didn't have anything left by 2002. The question of how that happened and what could have been done to prevent that mistake in the future is key.
2. The invasion and the proximate aftermath.
This should include both the military campaign itself, the initial occupation, the planning process for that - and a question of what went wrong, where the public sympathy from the Iraqi people was dissipated and how it could have been done better.
My view on the public evidence is that the UK and the US State Department both proposed a colonial solution, with the Allied forces seizing the existing reins of power, and then slowly removing the corrupt and the criminal officials, trying people, establishing the rule of law and establishing public order, and then slowly democratising.
I think there were some voices, above all the neocons - the Project for a New American Century types - that were calling for a total occupation, the replacement of all the political infrastructure by American/Allied occupation personnel, the commitment of a very large military to establish and maintain order and a large civilian presence to reconstruct Iraq and create a rule of law, followed by a rapid democratisation and then a handover of a reconstructed Iraq to the new government.
But, the dominant voice was, I think, Donald Rumsfeld, who saw maintaining Saddam-era institutions and personnel, even at a relatively junior level, as unnacceptable, but also regarded the price the PNAC people proposed as excessive. What he ended up doing was to propose the PNAC methodology and pace, but the State Department/FCO price and troop level.
Unsurprisingly, this was a disaster.
However, I wasn't privy to the internal discussions within the US and UK Cabinets on this, and I think we are owed the details of what really went on.
3. The ongoing occupation and the future
We need to know firstly where we stand; how much of Iraq has established order, how well is the rule of law operating, what kind of state is local government in, where are the Iraqi security forces up to.
Secondly, we need reasonable projections - which indicators are getting better, whch worse, at what rates, and what are the realistic prospects of turning the failures around.
Thirdly , we should look at the realistic options:
a) Reinforce the military in Iraq to enable them to establish and maintain public order more easily and to train the Iraqi forces more rapidly. Where would such reinforcements come from? Do we/the Americans need a more infantry-heavy military for these sorts of duties?
b) Keep on as we are and eventually hand over to the Iraqis - is this realistically ever going to happen? How long will it take?
c) Timetabled withdrawal / cut-and-run. Let the civil war happen, let Iraq get divided, then perhaps offer some reconstruction help to the replacement states.
d) Positive division: Publicly support self-determination for an independent Kurdistan, suggest a boundary between Sunni and Shia and help the mass population movement to get people onto their side of the border, then withdraw. This is nasty and would be seen as supporting ethnic cleansing, but may leave three stable states instead of one civil war. Opening up the boundary question in the rest of the Middle East will destabilise most other Governments, but given they are almost universally nasty dictatorships, is that necessarily a bad thing?