Leading article: The end of a misguided mission

Today, 2,985 days after the first British soldiers entered Iraq, the last contingent leaves. Since 20 March 2003, at least 100,000 Iraqis – shamefully, we can only calculate their number – and 4,769 American and coalition troops have been killed, including 179 British; 32,000 coalition troops have been wounded; violent deaths in Iraq are still running at an average of more than 300 a month; 2 million people have left Iraq, of whom 100,000 have returned, according to the Brookings Institute; coalition forces have used 250,000 bullets for every insurgent killed; the American taxpayer has spent $900bn (£500bn) in total, at the rate of $300m a day; the British taxpayer has spent a total of £8.4bn (£2.8m a day).

What, then, has been achieved, and what have we learned?

There is one statistic that can be entered on the credit side of the historical ledger. Number of dictators removed from control of Iraq: one. However unsatisfactory the state of Iraqi democracy, internal security and economy, the absence of Saddam Hussein is welcome. But one Iraqi minister in Nouri al-Maliki's elected government put it best a few years ago. He was grateful to the British, and he was hopeful about his country's future, yet, when asked if the invasion had been "worth it" to get rid of Saddam, a look of sadness crossed his face. "Worth it? No. How could it have been worth it?"

He reflected the opinion of his fellow Iraqis. In a BBC opinion poll in August 2007, 63 per cent of his countrymen said it was wrong, and 37 per cent right, that "US-led coalition forces invaded Iraq in spring 2003". The British public agrees. Before the war, in March 2003, 38 per cent approved of military action to remove Saddam – not that this was the ostensible purpose of military action.

No democratic country can hope to sustain a successful military engagement with domestic opinion so divided, because public support is so essential to forces' morale. Support was briefly massaged to 63 per cent during the war, in April 2003, but declined sharply thereafter, reaching 20 per cent five years later in March 2008.

The most important lessons for British foreign policy are pertinent this week, as Barack Obama arrives to discuss the future of the Afghanistan mission, among other things, with David Cameron. One of the most obvious mistakes of the Iraq episode was Tony Blair's belief that British interests were served by emphatic support for American policy.

When the US invaded Iraq on the false premise that this was a response to 9/11, Britain should have stood aside. This mistake led to another, which is that Mr Blair assumed that the Americans had a plan to stabilise and administer a country of 25 million people, the size of California, when it should have been apparent that they had no interest in such a thing.

What is surprising, perhaps, is that the Iraq disaster has failed to discredit the idea of liberal interventionism, which The Independent on Sunday supports as strongly as it opposed the war. Indeed, the experience of Iraq may have ensured that interventions will now be more cautiously based on worst-case assumptions. In Libya, for example, the situation was very different. There, Gaddafi was threatening a bloodbath in Benghazi; limited military action was justified, and the need for it was urgent.

In Afghanistan, on the other hand, the case for intervention, initially strong, ceased to carry weight some years ago. As we report today, much of the West's money to build a nation has been diverted into corruption. President Obama and Mr Cameron need to talk this week about scaling back their ambitions there.

The other lesson of Britain's part in Iraq is that this country has not lived up fully to its obligations to its armed forces. After many years, this newspaper's campaign to persuade the Government to give legal force to the Military Covenant is finally bearing fruit. The covenant enshrines the terms of the deal, by which we promise to give service personnel and their families the support they deserve for risking their lives.

In the end, none of the statistics can adequately sum up the loss of life suffered, mostly by Iraqis, as a consequence of a bad American decision, wrongly supported by the British government. And what will never be forgotten about this chapter in British foreign policy is this statistic about the stated reason for our going to war – number of weapons of mass destruction found: zero.