Menzies Campbell: Three years of war have been a disaster. Even the neo-cons know it. So what now?

Three years ago this month, we went to war in Iraq. But four years ago this month, when Jack Straw advised the Prime Minister about an invasion of Iraq, he said: "We have also to answer the big question - what will this action achieve? There seems to be a larger hole on this than on anything."

The Foreign Secretary's perceptive and prophetic words have a chilling resonance. He was echoing the concerns of many, but why did the Prime Minister, and indeed the whole Cabinet, go ahead regardless?

Recent weeks have seen a terrible intensification of sectarian violence. Extremists have deliberately sought to provoke internecine conflict, most spectacularly through the bombing of the al-Askariya shrine, which triggered scores of brutal reprisals as was intended.

Last week, Amnesty International described large-scale violations of human rights in Iraq, and the US State Department reported that "civic life and the social fabric remained under intense strain from the widespread violence, principally inflicted by insurgency and terrorist attacks". The US Ambassador to Baghdad was more candid: the invasion had "opened a Pandora's box" that could lead to a regional war and the ascendancy of religious extremists who "would make Taliban Afghanistan look like child's play". At the same time, a covey of senior neo-conservatives, arch-advocates of the war, now admit they got it wrong - seriously wrong.

With the transfer of the Iran dossier to the UN Security Council, and amid increasingly confrontational rhetoric from both Iranian and American politicians, we have to learn from the mistakes, misjudgements and mishandling of Iraq.

Coalition strategy on Iraq, both pre- and post-conflict, has followed a flawed pattern: a reliance on force; a belief that ends justify means; and a disregard for the lessons of history.

Recent disclosures show that President Bush and the Prime Minister had decided to go to war against Iraq months before the invasion, irrespective of the search for weapons, irrespective of diplomacy, irrespective of legality. As we now know, intelligence and facts were being fixed around the policy.

Under neo-conservative ideology, means are subverted to ends and principles to power: the consequence for Iraq was a grotesquely ill-judged and illegal war.

There was a scandalous lack of planning. The occupation was conducted with wholesale disregard for accepted principles of post-conflict stabilisation and counter-insurgency. Security forces were disbanded, creating a vacuum that invited upheaval; the UN was marginalised, with the US dominating the drawn-out political transition; there were and remain inexcusable delays in the restoration of essential services and the reconstitution of Iraqi security forces; and excessive use of military force and cultural insensitivity generated hostility and mistrust.

In responding to the insurgency, there has been a catastrophic failure to win the support of the Iraqi people: a strategy that won battles, but lost hearts and minds. The American experience in Vietnam, and the French struggle in 1950s Algeria, have shown that a strategy which relies excessively on military force and neglects the "cognitive terrain" cannot succeed. As the military expert, retired Brigadier Gavin Bulloch put it: "The strategic centre of gravity will be the support of the mass of the people."

Today, 14,000 prisoners are subjected to indefinite detention by coalition forces, without charge or trial, some of whom have been subjected to torture or other abuse. More than 35,000 have been detained since the war, over two-thirds of whom have since been released. Nothing could be better calculated to spread resentment. The closure of Abu Ghraib is a first but insufficient step; this gross and persistent violation of international law must come to an end.

There are currently more than 150,000 foreign troops in Iraq. Despite their important role in stabilisation and reconstruction work, we need to acknowledge that the troops are perceived as occupiers.

The latest figures from the Pentagon show the number of attacks on coalition troops reached a high of more than 550 a week; in a recent UK MoD poll, close to half of Iraqis believed the attacks were justified, and eight in 10 strongly opposed the troops' presence. We must accept that a clear, explicit exit strategy is imperative.

But an immediate withdrawal of coalition forces could precipitate even greater violence. In recent weeks, the insurgency has fanned the flames of sectarian strife, though the warring parties appear for the moment to have pulled back from the abyss of civil war.

The priority now is the formation of an inclusive government, founded on compromise, with prominent roles for Sunni leaders. Further measures must be taken to deliver what Iraqis so desperately need: personal safety and access to essential services. A national strategy is urgently needed to disband sectarian militias and reintegrate them into national forces. More must be done to train professional Iraqi security forces, root out militia and death squads that have infiltrated the forces of the Ministries of Defence and the Interior, and ensure appropriate ethnic and religious balance.

Regional powers could be encouraged to do more to stabilise and support Iraq. It is in no one's interest, least of all the countries of the region, that Iraq should become a failed state. Iran must be prevailed upon to desist from dangerous meddling.

The British government justified war on the basis of the threat from Iraq and the promise of benefit to Iraqis. But where there was no threat, there is now powerful insecurity; and Iraqis now suffer from militant extremism and violent upheaval.

The war undermined the authority of the UN and the post-Second World War framework of international law, largely fashioned by Britain and the US, more than any other single event since 1945. It has strained relations between the Western and Islamic worlds, and, as the intelligence services warned, has increased the terrorist threat.

It is breathtaking that after the deaths of more than 30,000 civilians and soldiers, including 103 British personnel, and at a cost to Britain of over £4bn, there has never been an inquiry into political decision-making on the war or its aftermath.

Sir Menzies Campbell is leader of the Liberal Democrat Party