This was a shambolic misadventure and we must learn from it

As the summer heat and dust rises over the Basra airbase the Union flag will be hauled down definitively, drawing to an end a six-year adventure from which our leaders, civil and military, must draw some urgent lessons. If they don't they are going to regret it - and pretty quickly.

Besides the audit of what the British presence in southern Iraq has achieved in the past six years, we must assess how and why we go about such ventures altogether. The epithet that most succinctly summarises the whole approach of the Blair government, in which Gordon Brown played lead consigliere, is that "casual" Blair, by his account of only this month, seems to have believed he could knock over the bad men of Baghdad with the ease he had knocked over the bad men of Kosovo in 1999.

In April 2003 he pitched some 40,000 British servicemen and women into southern Iraq in the belief that, like the Tommies going to war in August 1914, they would be home by Christmas. The facts on the ground soon revealed that the British and Americans were in a land and facing a people they did not understand and could not possess - and had landed themselves with huge and unexpected problems of governance and security. Despite the tactical successes of the American military surge crafted by General David Petraeus, these problems continue. There is scant political reconciliation and violence persists between Shia, Sunni and Kurd, and these have implications well beyond Iraq itself.

The British forces never had the numbers or resources to impose their will on the warring militias of the south. The parsimony of the UK government towards the forces, and the lack of appreciation of the challenges on the ground in Basra in the south, are matched by the more general failure of the New Labour governments to understand the complexities of armed intervention.

This shortcoming has been underlined just this week by Gordon Brown's stuttering presentation of his "new strategy" for Afghanistan, and the debacle of his handling of the claims for residence and welfare by Gurkha veterans. The Foreign Secretary in the meantime was away trying to fix a ceasefire in Sri Lanka, to head off the Gurkhas' fellow lobbyists at Westminster, the Tamils.

It is now time for a full and expeditious public inquiry into the British Iraq venture. Labour wants to kick this into the long grass until after the next election. They also want to restrict the area of inquest to the conduct of the campaign - presumably to let the politicians off the hook and blame the military. Given all the constraints, the British military did pretty well at an often impossible job, and sometimes better than that.

At issue is the current culture of Whitehall and Westminster, as none of the political leadership seem to understand the full cost and implication of defence and security and credible management of armed forces today. Messrs Clegg and Cameron have had a good day on the Gurkha issue, but they show no interest or understanding of defence matters, bar suggesting that they want the budgets cut. Mr Cameron's team wants an immediate cut of £1.5billion on the present budget, which extended over five years would mean no aircraft carriers, a smaller army, half the present RAF and almost no submarines.

Last week Tony Blair was unrepentant in restating his belief in the use of force for "humanitarian intervention".

Hopefully the coming inquiry will reveal the shambolic decision making, half-baked analysis and posturing that lay behind the Iraq misadventure - and how we might avoid it next time.