The Chilcot Inquiry shows a panel of experts examining how the decisions to go to war in Iraq were made, how justified those decisions were, and how effectively the post invasion and reconstruction plans were carried out. The aim will be to learn lessons about how to manage future conflicts.

Witness after witness has appeared, often making opposing arguments. Some argue that military action was legally justified. Others say the war defied international law. Some say it was a mistake to disband the Iraqi military. Others argue it was a necessary part of post-invasion reconstruction.

If the panel is looking for lessons that can be learnt in all of this they could do worse than to look at a little-known, but bloody battle, fought in an obscure part of Iraq on day three of the war. It was a battle that teaches us some fundamental truths about how to deal with future conflicts.

It was dawn on 23 March 2003, when US Marines approached the town of Nasiriyah in southern Iraq. Nasiriyah was a predominantly Shia town that had rebelled against Saddam Hussein during the first Gulf War. The intelligence suggested that 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 as the Marines moved into Nasiriyah they were attacked by massed numbers of Iraqi fighters few of whom seemed to be wearing military uniforms. Many were dressed in the distinctive black pyjamas worn by Shia muslims—the Shias that were supposed to be on the allies side. The American soldiers had been expecting to fight Iraqi soldiers. Instead they found themselves shooting at old men, women, even children.

Among the civilian fighters there were fanatical Sunni fedayeen troops and foreign fighters linked to Al-Qaeda. It was a potent and complex mix of insurgency; Sunni and Shia militants, Al-Qaeda inspired fighters and civilians, a mix that still causes such chaos in Iraq today.

But what was most striking at Nasiriyah 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 American war machine thunder into town. At worst, they ran to arms stashes, grabbed AK47s and took to the streets.
Eighteen Marines died in Nasiriyah that day in what turned out to be the bloodiest phase of the Iraq war.

Four days later the city was finally declared secure. One week after that, American forces triumphantly entered Baghdad and helped topple Saddam's statue. Everyone lauded the speed and efficiency with which coalition forces had fought their way to Baghdad. And the trauma of Nasiriyah was forgotten. And that was a shame, because it’s a battle which teaches us lessons for the future.

Lesson one: There is a limit to what armour and technology can do against a people who fight because they feel their country has been violated.

Lesson two: You play with long-standing national divisions at your peril. In Iraq, instead of coming together, Iraqis fell back into their faiths and tribes and ended up killing British and American forces, and each other.

Lesson three: War can ignite unpredictable forces. The invasion of Iraq unleashed a wave of Arab anger and resentment against the West which will not fade for generations.