One of the most difficult jobs for the US Iraq Study Group, which is due to report today, has perhaps been to pinpoint the moment that Iraq started to go wrong. How did scenes of joyful Iraqis hacking down Saddam's statue so quickly turn into images of car bombs, grieving mothers and burning helicopters? Some experts argued before the panel that it was a mistake to disband the Iraqi security forces. Others said there had not been enough troops on the ground. In most analyses, however, there is a tendency to treat the invasion and post-invasion of Iraq as separate entities. The invasion is generally portrayed as well planned and executed, the post-invasion strategy as ill thought-out, chaotic and undermanned; hidden somewhere in the months following the arrival of US forces in Baghdad there lies a magic moment when Iraq somehow began to descend into chaos.

In fact, the fight to get to Baghdad and the one coalition forces have been engaged in ever since have much in common. All the information about the nature of the trouble to come was apparent from the first days of the war. If lessons learned then had been incorporated into military and political thinking, it would have injected a much-needed dose of realism at an early stage.

Those lessons were best synthesised in a little known but bloody battle, fought in a lesser known part of Iraq on day four of the war. It was a battle that America nearly lost.

It was dawn on March 23 2003 when marines from Task Force Tarawa approached the town of Nasiriya in southern Iraq. They had been assigned a routine manoeuvre, taking two key bridges to open up a route to Baghdad. Nasiriya was a predominantly Shia town that had rebelled against Saddam Hussein during the first Gulf war. US intelligence suggested that as soon as the Americans rolled into town, the city's defenders would lay down their weapons and, as one marine commander expressed it, "put flowers in our gun barrels, hold up their babies for us to kiss and give us the keys to the city".

But when Task Force Tarawa's lead units reached the outskirts they came across the burnt out remnants of several vehicles of the US army's 507th Maintenance Company. A captain in the 507th told wide-eyed marine commanders how his convoy had taken a wrong turn at night, driven into Nasiriya and been attacked by Iraqi fighters. Several US soldiers were still missing, including a young army private, Jessica Lynch.

It was her fate that attracted attention in the days following. But it's what happened to Task Force Tarawa that is most instructive about the nature of the Iraq war and about what life would be like once coalition forces got to Baghdad.

As marine units moved into Nasiriya they were attacked by massed numbers of Iraqi fighters. To the surprise of marines on the ground, few of the Iraqi combatants seemed to be wearing military uniforms. Many were dressed in the distinctive black pyjamas worn by Shia Muslims, and much of the gunfire came from dwellings flying black flags, denoting them as Shia homes. And yet the Shias were supposed to be on the Americans' side.

What's more, as the marines were drawn into a raging battle in the city centre, more and more people came out of ordinary homes to take up arms. One group of young marines, separated from their unit and forced to commandeer a house, found themselves under attack for several hours from what appeared to be armed civilians. They had been expecting to fight Iraqi soldiers. Instead they found themselves shooting at old men, women, even children.

Of course, there were Sunni-led fedayeen troops and foreign fighters who fought that day. But some who picked up weapons were also civilians intent on defending homes against foreign invaders. The potent and complex mix of insurgency - Sunni and Shia militants, civilians and foreign fighters - that causes such chaos in Iraq today was already apparent during the battle of Nasiriya.

Intelligence about the terrain was also sorely lacking. Marine tanks spearheading the manoeuvre took a route that led to a bog where they sank, mired uselessly in thick mud, while the battle raged. This is more than just a metaphor for coalition forces post-invasion, in towns such as Falluja and Samarra. It was the product of a rush to arms without adequate intelligence or planning.

At one stage in Nasiriya, in a friendly-fire incident, US planes fired at marines on the ground, killing up to 10. Radio communications repeatedly failed. Units lost contact with each other and went missing in the city. Faced with an increasingly determined enemy, marine commanders thought they might just lose the battle. It showed another truth, obscured during the march to Baghdad, that has become strikingly apparent since. There is a limit to what armour and technology can do against a people with faith, who fight because they feel their country has been violated.

There were other incidents in Nasiriya, minor at the time, that foreshadowed events that would become an international embarrassment. At one point, a marine commander came across a gruesome scene: young marines, standing over a pile of Iraqi corpses, taking photos of each other, thumbs up and grinning inanely. One year later the first photos of US soldiers grinning over the bodies of abused prisoners emerged.

But what was most striking at Nasiriya in those very early days of the war was the absence of that grand coalescence of freedom-deprived Iraqis who were to come forward and support coalition forces. At best, civilians stood by and watched the US war machine thunder into town. At worst, they ran to arms staches, grabbed AK47s and took to the streets. And that did not bode well for life in post-invasion Iraq. Instead of coming together, Iraqis would fall back into their faiths and tribes and end up killing coalition forces, and each other.

Eighteen marines died in Nasiriya that day in what turned out to be the bloodiest phase of the Iraq invasion. Four days later the city was finally declared secure. One week after that American forces triumphantly entered Baghdad and toppled Saddam's statue. There was much praise for the speed and efficiency with which US forces had fought their way to Baghdad. And the trauma of Nasiriya was forgotten.

And that was a shame. If the intelligence from Nasiriya had been gathered, recognised and analysed more soberly early on - instead of trampled in a rush of triumphalism - coalition forces might have learned useful lessons for the reconstruction of Iraq: the limits of military power; the importance of understanding the complexity of a place and its people; the perils of underestimating an enemy. Instead, George Bush stood on the USS Lincoln and made his hubristic speech announcing the end of combat operations under a banner that read "Mission Accomplished".

The battle of Nasiriya taught that there was, contrary to first appearances, no simple route to Baghdad. It should also serve to remind those in London and Washington that there will be no simple route out of it, and that the next move Britain and the United States make in Iraq should be handled with less haste, more care, more sensitivity, and more humility.

· Tim Pritchard is the author of Ambush Alley: the Most Extraordinary Battle of the Iraq War tim@timpritchard.com