All of us, at every turn in our life, encounter circumstances in which there are severe limits to our ability to intervene. We feel no shame in this sane and commonplace response. “There's only so much I can do,” we say.

If as individuals we so readily acknowledge private incapacity, how is it that, when we act as publics, parliaments, nations, armies, or indeed newspaper columnists, we find such simple truths so hard to acknowledge? Nations do, of course, come up against limitations. Reality rebuffs. But we fight shy of the language in which to talk - and think - about impotence. And so (like those hyperactive battery-powered puppies they sell in novelty shops) we bounce around the pen within which fate confines us, changing direction only when we hit a wall, then heading off with mechanical yaps towards another one.

Forgive me for writing like this yet again, of Afghanistan. None of us can know whether the situation is beyond retrieval but we surely sense that we British - never mind about America, or Italy, or Canada, Germany or France - are at the limit of what we can achieve by force. It is no good sending any more troops: we haven't any to spare, and the force we already send to Helmand province is overstretched. In Paddy Ashdown we have offered the best imaginable possibility for a figure capable of knocking heads together, and the Government of Hamid Karzai has rejected him.

Three recent reports - most worryingly one from Oxfam - have painted a picture of a failing state. Inch by inch we are being edged into keeping thousands of troops permanently parked in a barbarous place, in the open-ended support of a puppet government led by a man who wears elegantly tailored clothes and speaks nice English but whose writ hardly runs.

And now the Americans are demanding more troops from Nato. Well, good luck to them. Perhaps they will persuade the French to do a little more; maybe they can stop Canada from carrying out its threat to pull back. But the starting point for a British Foreign Secretary is that in terms of boots on the ground, we British are at our limit and losing confidence in our usefulness.

There is, I concede, no immediate crisis to respond to. People tend to think that brinks, thresholds, Rubicons, cliffs' edges and forks in the road are where historic decisions are called for and statesmen are proved. But doldrums, paralyses, slow-drifting currents, slow roads going nowhere - times when no decision seems urgent and a vaguely unsatisfactory situation can safely be allowed to drag on - can be greater tests of mettle than emergencies. The politician with the guts and brains to say “It can't go on like this” and convince Cabinets and mandarins who might have preferred a long, expensive drift - these are greater heroes than men who, cometh the hour, do what plainly has to be done.

This Afghan drift could be the moment for the British Foreign Secretary, David Miliband.

“British Foreign Secretary” - it sounds almost old-fashioned, doesn't it? As though these creatures could dominate Cabinet any more. As though they stood up to prime ministers and argued on equal terms, as they used to. Perhaps Mr Miliband should try.

Sooner rather than later this year he should go to Afghanistan. When he does, all the pressure will be for a purely staged visit. He serves a Prime Minister whose bent is to procrastinate, who would be horrified if his minion returned with clear impressions or changed conclusions. Mr Miliband will be pressured into an itinerary that is all quick flips by helicopter for photo-ops that imply an acquaintance never made.

But why not resist? Why not insist that on landing in Kabul his feet barely touch the tarmac before he is off into the deserts, the poppy fields and the mountainous yonder. Kabul is not Afghanistan.

In Helmand he should walk in the burgeoning poppy fields (just beginning their growing season). Far from Kabul he should visit the provincial camps of IDPs (internally displaced persons), where the relief funds he has signed off in London are nowhere in evidence, and where the Taleban recruit. Let him ask whether set-piece battles in Helmand are the most efficient way to confront al-Qaeda. Let him sit, as I did, in the sweet stink among the marijuana fields near Mazar-e-Sharif.

Perhaps he could go in time to stop the execution of Sayed Parwez Kaambakhsh, the 23-year-old journalist whose conviction for blasphemy has just been upheld. And if his officials insist that Britain has no such leverage, and it is unsafe for ministers to visit any of these places, then let him draw his own conclusions.

We are failing in Afghanistan, and while we fail, real British servicemen die. Not for them the luxury of hypotheticals. Not for them the philosophical certainties of armchair commentators, nor the consolation that the moral and strategic argument did clearly call for intervention. For them the sacrifice is pointless unless it works. For how long should we subjugate the observation that it isn't working to the conviction that it would be an excellent thing if it did?

Mr Miliband, whom I hardly know, has so far proved a journalist's politician of some class. He is quietly fun over the silverware at supper. His brains are apparent. His instincts appear civilised. There is no wild glint in his eye, no stubborn set to his jaw. He listens. With him, you feel yourself in the presence of a tolerant, keen and mildly amused intellect, sympathetically cynical, at ease with himself and with you, and mellowly conscious of the ambiguities of life: of the many sides to every question. There is almost a wink.

Journalists adore such politicians. Echoing our own political nomadism, they flatter us. We leave convinced we have met a future leader. Only the tiniest reservation in the back of the mind whispers the question: is he in politics to do anything? Does he know what he wants to do?

I am not - not by any means - urging the conclusion that Mr Miliband does not know. Chris Patten was in his mould, but when his turn came he was decisive about what could - and decisive about what could not - be done; and brave.

Courage for a British minister in Afghanistan in 2008 starts with making his own estimate of our chances and scope - where we might succeed, where we are failing - and then acting on it. He should not see the status quo as the default option.

The default option is that we pull out altogether. And others have to believe this. I doubt at present they do. They assume good old Britain will stick around in Helmand and stew, losing a soldier or two from time to time, and sometimes a Jeep-load; losing a town here and retaking another there; and making a token show of supporting a failing poppy-eradication programme come what may - unless and until the Americans change their minds.

The whole world assumes this. It is an assumption that Mr Miliband needs to upset.

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