The ghost of Tony Blair could be heard in the House of Commons on Friday, when David Cameron made a statement about the Algerian hostage-taking crisis. "We will stand with the Algerians in their fight against these terrorist forces," he said. "Those who believe there is a terrorist, extremist al-Qa'ida problem in parts of North Africa, but that it is a problem for those places and we can somehow back off and ignore it, are profoundly wrong. This is a problem for those places and for us."

As the 10th anniversary of the disaster of the Iraq war approaches, and as British troops are still taking casualties in our long overstayed mission in Afghanistan, how uncanny it is to hear a British prime minister using such similar words with only the names of the countries changed.

That may be partly because Mr Blair was not wholly wrong about Afghanistan, at least. The original intervention in Afghanistan was justified. The mistake was to think, once the Taliban had fallen, that it was our responsibility to construct a workable democratic society – from the roads and power stations up.

Therein lies the lesson for Mr Cameron. It was the lesson of Vietnam. The danger is that early decisions may lead to further decisions taken by default, and under less scrutiny. That is why the Prime Minister should be careful about sending those two transport planes to help the French intervention in Mali. That is one of the ways in which countries can take the first innocuous steps into a quagmire.

This is part of a broader lesson, which is that military action, even for the best of motives, can have unforeseen consequences, either because they were unforeseeable or because they were not thought through. The Iraq invasion, far from denying opportunities to terrorists, did exactly the opposite. It created precisely the kind of "ungoverned space" about which Mr Cameron is now worried in the Sahara, in which new groups inspired by al-Qa'ida now operate.

The law of unintended consequences applies in cases where the case for military intervention is far stronger than it ever was in Iraq. This newspaper supported the no-fly zone – authorised by the United Nations – over Libya in 2011, and is proud of the UK's role in enforcing it. Libya's progress since Gaddafi's fall, as John Rentoul notes opposite, is partly to Mr Cameron's credit.

Yet Libya's liberation has plainly sent weapons and soldiers into the Saharan corridor of instability. It was not the cause of civil war in Mali, but it has not helped, and it would seem to have contributed to the raid on the Algerian gas plant.

It is at this point that the Prime Minister should prefer caution to rhetoric of the domino theory and the spread of a violent Islamist ideology against which the democratic world must stand. As our writers explain today, the politics of this corridor are more complex than a simple attempt by al-Qa'ida to find new franchises for its terrorist brand. Ethnic and sectarian conflict merge into banditry across that whole strip of the continent, and direct European military "assistance" is likely to be anything but, except in the shortest of terms.

Mr Cameron, learn the right lessons from Mr Blair, not just how to make fine declamations of apparently moral sentiment. Proceed with care.