As the troops marched to their doom in the First World War they sang a characteristically British military anthem. To the tune of Auld Lang Syne, they intoned “We’re here because we’re here because we’re here because we’re here.”

Once human beings are committed to an endeavour and have made sacrifices in its prosecution, they are strongly disinclined to subject it to serious review. The idea of abandoning the course they are on before they have reached its end becomes unthinkable. Far better simply to plough on. That is how people can cheerfully sing that they don’t know what they are doing, while continuing to do it. Even if doing so might lead to their death.

However strong an instinct this may be, it should be equally strongly resisted in the case of our latest war. In Afghanistan, it is simply not good enough to be there because we’re there because we’re there. The cost we have paid in blood and treasure demands that the questions never stop and the thinking never ceases.

So this week The Times will be investigating five sets of questions about the conflict in Afghanistan. The first is whether the conflict is winnable. On the answer to this, all the other answers depend. There is no point pressing on with a war that can never end successfully. This, however, immediately raises the question of what success means. What are we fighting to achieve? Would it be satisfactory merely to ensure that Afghanistan did not become, as it was, a terrorist base? Or is that too modest an objective? Would it be a betrayal to leave the Afghan people with an oppressive government that agrees to oppress only its own population?

Beyond preventing terrorism and, perhaps, ensuring some form of government by consent, do we have other objectives? What, for instance, about ending, or at least reducing, the drug trade? Determining whether to press on in Afghanistan and what form our intervention should take depends on how tightly the mission is defined. The need for a definition is urgent.

Equally urgent is a resolution to the crisis of legitimacy of the Afghan Government and its President, Hamid Karzai. The President’s decision to use government resources to inflate his vote in the recent elections was a disaster. It is only the latest one for which he has been responsible. When he first arrived on the scene, Mr Karzai seemed part of the solution, now he appears part of the problem. It is not only the ballot stuffing, but also corruption and incompetence. So should we be pressing merely for a further round of voting, or for a more radical reappraisal of the Afghan consititution?

This leads to the thorny issue of the Taleban. Should there be an attempt to pacify them, by integrating them into government? Or would that be a form of defeat? A great deal depends on making a sensible assessment of their strength.

Once we have decided what we are hoping to achieve in the conflict and broadly how to do it, two sets of questions remain. There are questions about the resources we are prepared to send and there are questions about the extent of our political commitment.

The biggest resource question regards troop numbers. Is it true, counter-intuitively, that the more troops we send, the fewer will be killed? And we cannot send soldiers to war while not sending the equipment they need to succeed. British soldiers are being blown up by landmines because we haven’t enough helicopters. But how many troops, and how many helicopters, are enough? Is “enough” even possible?

Finally there is the political leadership of the war. In each of the participating countries, opposition is now rising. A war that might be won abroad, could be lost at home. It must not be.