Have the lessons of Iraq really been learnt?

Ten years ago this weekend, a million people marched through the streets of London in opposition to the Blair government's seemingly unstoppable journey to war in Iraq. It is difficult now to imagine such emotional outpouring as there was in London that day. But such was the opposition to the Prime Minister and his clear determination to join with President Bush in military action against Saddam that people of all political parties and none, and from every economic and social background, came together in support of the maxim "not in my name".

This, remember, was the Prime Minister who had embraced the emerging legal doctrine of humanitarian intervention and shown with considerable success its effectiveness in Sierra Leone and Kosovo. But who, a few weeks after the march, in one of the most powerful speeches of recent years in the Commons, sought without legal justification the endorsement of his unqualified support for the ambition of George W Bush. For all his brilliance, Tony Blair could not get the wholehearted support of his own party in the division lobby. Nor could he carry public opinion with him.

A decade ago, in this newspaper, I wrote about the mass movement of popular opposition to military action which the Prime Minister had inadvertently mobilised. I acknowledged that Blair was a conviction politician, and that "in that very conviction lies risk.... Indeed, it is that very certainty that makes public opinion in Britain so uncomfortable with George Bush. The British are not naturally crusaders."

And so it was we went to war.

It is hard now to find anyone who will justify British participation in the American-led venture; hard, too, to find any enthusiasm for it in the United States outside the ranks of neoconservatives. At best, you hear the assertion that the world is a better place without Saddam Hussein. But as recent events have shown, violence in Iraq continues and the government remains fragile. For Britain, the last word may be provided by the Chilcott inquiry, now expected to report by the end of the year.

What purpose is served by raking over the events of 10 years ago? The principal actors are no longer on the stage and the world has moved on. But contemporary challenges bear comparison with the events of 2003.

Parallels are dangerous tools in the study of foreign affairs. But current similarities cannot be ignored. In justifying the provision of British assistance to the French in Mali, David Cameron's tone was not pragmatic but evangelical and Churchillian. One cannot help but wonder if the success of British and French military intervention in Libya provided an incentive to speak thus. To be the "heir to Blair" in foreign policy would surely be an aspiration too far.

If Libyan success provides encouragement for engagement in the Maghreb, it is as well to recognise the extent to which the Anglo-French effort was heavily dependent on American support in such matters as communications, intelligence and logistics.

President Obama's inauguration speech, with its concentration on domestic issues, hardly suggests that such assistance would be readily forthcoming in future. And with defence budgets throughout Europe being reduced and our own defence resources under continued pressure, a decades-long engagement would be impossible to sustain financially, even if political support could be achieved. When, in response to the Prime Minister, some have queried the level of resources available to support his ambitions they have been assured that, in the future, our armed forces will be "leaner, fitter and more flexible". But this is a language all too familiar from defence ministers over the past 25 years, as defence budgets are squeezed by the Treasury with the inevitable impact on capability.

So what is to be done to protect British interests against instability in North Africa? First, turn down the volume. Do not make pledges that raise expectations and which cannot be realised. Do not describe the protection of our interests as if it was some Manichean struggle. Do not make demands on our armed forces which stretch their reach and damage their morale. Invest in those areas in which we have niche capability and proven excellence like intelligence and special forces. Don't believe that we can play special constable to America's policeman. Support democratic governments, but do not intervene in civil wars. Encourage European cooperation, but be assiduous in urging our allies to maintain adequate defence and security budgets. Understand the region and the distinctive characteristics and individual histories of its countries. Be resolute in the support the rule of law and human rights.

What do all these have in common? Unlike Iraq, they will have the support of the British public. That approach will not eliminate risk, but will minimise it.

We lost moral authority as a result of our engagement in Iraq and shed a great deal of blood. Coupled with our commitment to Afghanistan, where these costs have been even greater, the appetite of the British public for foreign engagements is dulled. In times of domestic austerity, it is tempting to look abroad for an assertion of our continued relevance. But Iraq changed the political imperative. No PM could now venture to go to war without the endorsement of a vote in the Commons, as British involvement in Libya showed. To obtain such support, the cause must be just, legal and winnable.

But even if these factors were present in the minds of MPs, public support would still be essential. Many of those who marched 10 years ago were frustrated that their unprecedented demonstration failed to prevent arguably the biggest foreign-policy mistake since Suez. But they can take comfort from the fact that their march irreversibly changed the terms of trade. For "not in my name", read "not without my support".

Sir Menzies Campbell was leader of the Liberal Democrats 2006-07