Gordon Brown has only put off the inevitable day of reckoning over Iraq. Yesterday he rejected calls from David Cameron and Nick Clegg, and the nationalist parties, for an early inquiry, arguing that this would not be considered until British troops were out of the country, now due next July.

However, not only have inquiries been held during wartime in thepast (for instance, over the Dardanelles operation during the First World War) but any investigation now would have no impact on the remaining British military role over the next half-year. This is partly about delaying any report until after the next election. But the risk for Labour isthat the terms of reference wouldthen be set by a new Conservative government, and they would be tougher.

Timing aside, there are two questions: the structure of an inquiry and what is examined.

Most inquiries are initiated by the executive. But the Public Administration Committee of the Commons, chaired by Tony Wright, has sought to restore a role for the legislature by pressing for parliamentary commissions of inquiry in future over the conduct of government.

There are several practical issues about how to deal with secretmaterial or excess partisanship (as undermined the Marconi shares inquiry in 1912-13, letting Lloyd George off the hook). But many of these difficulties can be overcome: for instance by ensuring that its members are Privy Counsellors.

Defining the remit is fraught, mainly because there is so much to examine. The main public controversy has been over the legal issues and the intelligence background (well covered in the report by Lord Butler of Brockwell in 2004).

I am sceptical about whether much new would be learnt about either topic.

In substance, we already know why Britain went to war. The real question is the how: what Sir Jeremy Greenstock, the former British Ambassador to the United Nations and then envoy to Baghdad, has called “woefully inadequate planning” for the aftermath of the invasion. It was the glaring failures there that turned Iraq into such a long-drawn-out horror story.

However, any wideranging inquiry into either the months leading up to the invasion or the subsequent occupation would have to examine the often fraught relations between London and Washington, and on the ground between British and American commanders and diplomats (though much improved over the past 18 months).

This would involve looking at private conversations between Tony Blair and President Bush, which neither leader would be keen toreveal. One senior official told me that the only conclusion would be: “Don't go to war with the Americans.”

However, any such inquiry could have a damaging impact on relations between Downing Street and the White House, despite the fresh start to be promised in January by President-elect Obama.

No wonder the Prime Minister was emphasising the more positive aspects yesterday. But the grievous errors of the past cannot be ignored for ever.

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