Paddy Ashdown, the former High Representative in Bosnia-Hercegovina, has been comparing Iraq with other international interventions, ranging from Germany in 1945 to El Salvador in the 1990s, for a BBC series.
The US has alienated Iraqis who initially welcomed the invasion
Iraq is a poor advertisement for intervention by the international community.
The level of violence there has been horrendous since the fall of Saddam Hussein and daily life remains enormously difficult for most citizens.
For the most part, the presence of foreign troops and administrators is widely resented.
The tragedy is that the Americans and their allies had a huge wealth of experience from other countries on which to draw and enjoyed wide popularity among Iraqis at the start of the occupation.
But they ignored every past lesson, lost the golden hour after the fall of Saddam's statue and with it the support of the people they were trying to help.
A place for the UN
I am convinced that, despite the disaster of Iraq, the international community has an increasingly important role in the world's hotspots.
In an ever more interdependent world, bodies like the United Nations and the European Union are inevitably being drawn into helping to resolve conflicts and rebuild shattered nations.
And despite the high-profile failures, we do know how to do this. We have succeeded in post-conflict reconstruction more often than we have failed and the world is a safer place because of it.
One estimate is that intervention by the UN has halved the number of wars and more than halved the number of casualties in conflicts round the world since the end of the Cold War.
Post-war Germany's experience
The Allied occupation of Germany after World War Two, particularly in the British zone, contained a great many lessons for the future.
Leader of UK Liberal Democrat party 1988-1999
Knighted in 2000, became Baron Ashdown of Norton-sub-Hamdon in 2001
Named High Representative for Bosnia and Herzegovina in 2002; left in 2006
Named chair of Northern Ireland parades review body in May 2007
The first instinct was to punish the Germans. British troops were forbidden even to speak to civilians. Huge swathes of German industry were dismantled.
And the authorities embarked on a vast programme of de-nazification, removing tens of thousands from their jobs. Germans were heard saying that they looked forward to the Fifth Reich, because the Fourth was as bad as the Third.
The policy was disastrous, and Germany was only saved because of the Russian threat. The Allies soon came to realise that they needed Germany as a partner against the USSR. De-nazification was abandoned.
The allies reversed their policy of dismantling German factories and replaced it with the Marshall Plan and a strenuous programme to encourage German economic growth. By 1949, Germany was on the way to its economic miracle.
The lesson was clear. You cannot run a country if you remove most of the people with administrative experience.
You must do everything in your power, by harnessing the efforts of the local population, to restore basic services like water and electricity, so that people can see that their lives are being improved.
None of this was done in Iraq.
Disbanded and disgruntled
The Iraq war has been described as a catastrophic success. Victory was won so quickly that very little thought and almost no planning was devoted to managing the peace.
The UN played a successful part in stabilising San Salvador
Officials at the US state department had done some preparation, but a matter of months before the war started they were cut out of the loop and reconstruction was entrusted to the defence department.
So those on the ground lacked any experience of the tasks before them. They in turn were micromanaged from Washington by officials who knew even less.
De-baathification in Iraq mirrored the early errors of de-nazification in Germany.
As former UN Secretary-General Kofi Annan told me, it was a colossal mistake, a judgment echoed by the first American administrator in Iraq, Gen Jay Garner, and many others.
The whole Iraqi army was disbanded. The result was a double whammy.
Not only was there a dangerous security vacuum, rapidly filled by the looters and the insurgents, but there were also large numbers of unemployed ex-army officers, smarting from the loss of prestige and income, all too happy to join the opponents of the new order.
Without a plan
After a conflict like the Iraq war, the priorities should be first security, second restoring services and third getting the economy going as fast as possible.
All require planning and substantial resources, often a great deal more than needed for the war itself.
Yet time and time again, the international community has found itself effectively running a country with no plans in place.
When my predecessor Carl Bildt, the first High Representative in Bosnia, arrived there after the conflict, all he had was an envelope full of money.
He had no office, no transport, no telephone and no blueprint for carrying out his task.
There was another fundamental mistake in Iraq.
For far too long, the US had failed to bring in neighbouring states like Syria and Iran, which had an enormous stake in the stability of the region.
Yet 12 years earlier, the UN had been successful in stabilising El Salvador after an exceptionally bloody civil war, in large part by involving neighbouring states which were desperate that the conflict did not spill over its borders onto their soil.
Again the lessons were there but completely ignored. Kofi Annan pointed out to me the inconsistency in the American view that, while the US itself can have a legitimate national interest in Iraq, a neighbouring state like Iran cannot.
So the conclusion from Iraq is not that the international community should never intervene again. It is rather that future interveners should study history and prepare carefully.
History teaches us these lessons for the interveners: leave your prejudices at home, keep your ambitions low, have enough resources to do the job, do not lose the golden hour, make security your first priority, involve the neighbours.
And remember that post-conflict reconstruction is not for the faint-hearted. It requires toughness, strategic patience and a willingness to stay until the task is finished.
You can hear the first part of Lord Ashdown's radio series Winning the Peace at 0805 GMT on 11 June on BBC World Service. His book Swords and Ploughshares, about conflict resolution, was published recently.