Britain's long campaign in Iraq achieved almost nothing. The 46,000 UK troops who took part in the initial invasion in 2003 helped to overthrow Saddam Hussein – but this would have happened even if they had stayed at home.

The decisive moment in Britain's intervention came on 24 June that year, when six soldiers were killed in the grim, dangerous Iraqi marsh town of Majar al-Kabir, a centre of resistance to Saddam. Local people were asking what British troops were doing using dogs to search for weapons in their houses. The killings revealed for the first time the dangers facing our forces in southern Iraq. All the people in the town were Shia Arabs who had suffered under Saddam. They welcomed his overthrow but did not want to be occupied by Britain, or anyone else. They saw UK forces as colonial occupiers, little different from the British armies that occupied Iraq in the First World War.

The British Army in Basra was never able to gain full control of the whole city. One intelligence officer who spent a long time in Basra said the problem for the Army there was that it had no real allies. "We used to patronise the Americans and say we had long experience of counter-insurgency gained in Malaya and Northern Ireland," he said. "But in those places we had the backing of a large part of the local population, while in Basra nobody really supported us."

For years, Tony Blair and a succession of defence ministers spoke of training Iraqi forces, but the central problem was not military expertise but loyalty. Many Iraqi police was in league with militias, either because they were paid or intimidated or for patriotic reasons. On one official British Government press tour, a supposedly pro-British police chief would only meet visiting journalists at night in a warehouse away from his station, and then only on condition that officers he worked with were not told he had done so.

British commanders generally had a much clearer picture than the Government of the quagmire in which they were landed. There was never any sign that Mr Blair took on board what was happening in Iraq. British generals were often openly critical of the massive use of force by the US Army, such as in the siege of Fallujah in 2004 when its artillery fired heavy shells into densely populated civilian areas. There was seldom any sign that American commanders took British reservations seriously.

Did the Iraqi government's surprise success in sending its army into Basra in March show that the British could have done the same thing earlier? Did British commanders exaggerate the strength of Shia militias? The answer is that Iraqi soldiers can do things in Iraq that British soldiers cannot because they are not foreign occupiers.

This is more than a question of local public opinion. It was Iran which mediated an agreement under which the Mehdi Army fighters loyal to the Shia cleric Muqtada al-Sadr were stood down to allow the Iraqi army to take over their strongholds. Iran would not have done this if British forces alone had tried to fight their way into the city.

Britain did not gain anything from its intervention in Iraq, but neither did it lose very much apart from the young soldiers killed or maimed in a slow-burning and unwinnable guerrilla war. Above all, it was an unnecessary war.