Ever since Alexander the Great was wounded by a Pashtun archer’s arrow in 327BC Afghanistan has been known as the graveyard of empires. It is now becoming increasingly clear that it could be the cemetery of political careers as well — although that is perhaps an inappropriate metaphor given that more than 230 real bodies have so far been buried during the current campaign.

Yesterday Gordon Brown had to apologise for misspelling the name of a soldier in a condolence letter to his mother. But the real problem is not the Prime Minister’s handwriting, but the Government’s failure to persuade the public that this is a war worth fighting.

According to a recent opinion poll, 64 per cent of Britons think the war in Afghanistan is unwinnable and 63 per cent believe troops should be withdrawn as soon as possible. With films such as Charlie Wilson’s War and The Kite Runner, Hollywood has popularised the bloody history of Afghanistan, embedding in the public consciousness the impression that this is a failed state that destroys those who interfere.

In Washington, political tensions have burst into the open as Barack Obama agonises over sending more troops. Joe Biden, the Vice-President, has called for a switch to a counter-terrorism campaign, based on blitzing training camps from the air rather than trying to beat an insurgency on the ground. Divisions are emerging in this country too.

Last week, Kim Howells, the former Foreign Office minister called for troops to be withdrawn. Geoffrey Robinson, the former Paymaster General, recently condemned the “futility” of Operation Panther’s Claw. Although publicly the Cabinet is united behind the Prime Minister, privately there are growing concerns about the lack of clarity over the purpose of the military campaign. The failure to rerun the fraudulent election has solidified the anxiety.

“It doesn’t seem to me that a political strategy based on propping up a corrupt and discredited President Karzai in Kabul is credible,” one Cabinet member says. Others criticise Mr Brown’s communication skills. “We are failing to get any message of justification across and so the polls go from bad to worse,” an aide says.

There has not, I am told, been a full Cabinet debate about Afghanistan recently — although there are almost weekly updates from Mr Brown and Bob Ainsworth, the Defence Secretary. Peter Hain, the Welsh Secretary — who is replacing Robin Cook as the voice of the thoughtful Left in Cabinet — has twice used these to raise questions about the strategy. On the BBC’s Question Time last week he hinted at his concerns when he said: “We cannot win this fight solely by military means.” He is not the only one who is worried.

Among ministers and at No 10 there is a growing desire for a high-powered UN consul, who could keep President Karzai in check. “The fact is this campaign will never be winnable unless there’s a legitimate Government in Kabul,” says a senior diplomat who was involved in the plans after 9/11. “You can’t send people off to get killed defending a system which is doing nothing to defend itself.”

It’s not only Labour that is feeling the pressure. There are tensions in the opposition parties too. Last week Nick Clegg hinted that the Liberal Democrats might support troop withdrawal — the day after Paddy Ashdown said that failure in Afghanistan could let extremists get hold of Pakistan’s nuclear bomb.

The Conservatives are also divided between foreign policy idealists, who want to export liberal democracy around the world, and pragmatists who think it is time for Britain to take a narrower view of the national interest. Liam Fox, the Shadow Defence Secretary, and Michael Gove, the education spokesman, are staunch Atlanticists and supporters of military intervention — “neocons” to their critics, although Dr Fox prefers “neo-realist”.

When the Shadow Defence Secretary met General Stanley McChrystal recently he made clear that a Conservative government would be more willing than Labour to increase the number of British troops. “Imagine if Churchill had said — ‘things aren’t going well in the opinion polls’,” Dr Fox told me last night. “If we are forced out that would be a shot in the arm to jihadists everywhere.”

Other Shadow Cabinet ministers think that the priority must be to hand over to the Afghan Army and bring British soldiers home. “There is a significant section of opinion in the parliamentary party that wants the thing over as soon as possible — you can call it isolationist or realist but it is there” one frontbencher says. George Osborne, an instinctive neocon, has, colleagues say become an “economic realist” who is struck by the cost of the war at a time when he must save billions. William Hague hovers between the two positions.

Last week David Cameron showed his pragmatism on another area of foreign policy — Europe. For him power matters more than ideology and he is instinctively suspicious of the philosophy of liberal interventionism. Shortly after the Afghan elections, he was overheard condemning the “naked” fraud. In recent months he has been influenced by an old friend from Eton, James Fergusson, whose book A Million Bullets, based on conversations with ordinary soldiers, makes clear how gruelling and difficult Afghanistan has been.

“David is firmly in the realist camp,” a Shadow Cabinet minister says. “He has a clear sense of the limited objective of foreign policy, that all this nation-building is incredibly difficult. He’s not going to have a Blairite objective of reshaping the world.”

This week it is Mr Brown who is feeling the heat from the 21st-century version of the Great Game, but Mr Cameron could soon find himself having to reconcile different instincts in his party. Then the Pashtun archer’s arrow will be aimed at him.

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