"War is always about betrayal," the Pulitzer-Prize-winning war reporter Chris Hedges once wrote. "Betrayal of the young by the old, of idealists by cynics and of troops by politicians." Nowhere is this betrayal better illustrated than in Afghanistan, where Britain is fighting its fourth war (the previous conflicts being those of 1839-42, 1878-80 and 1919).

We blundered into the country ill-prepared, underequipped and offering ever-shifting justifications. Was it counterterrorism? War on narcotics? Nation-building? The liberation of women? Crucially, we also lacked an exit strategy. The deaths have since piled up.

The fourth Afghan war is a cross-party betrayal. It was a Labour prime minister, Tony Blair, who paid the "blood price" to join with George W Bush in his invasion of Afghanistan in October 2001. He did so with the support of the Conservatives and Liberal Democrats.

It was a Tory premier, David Cameron, backed by his Lib Dem deputy, Nick Clegg, who promised a complete withdrawal of UK combat forces - but only in 2015.

Every Wednesday at the start of Prime Minister's Questions, we are treated to the nauseating spectacle of the pro-war PM and the pro-war leader of the opposition, Ed Miliband, paying sombre tribute to the latest British soldiers to have died, before they get on with the much more serious business of tearing strips off one another.

Glazed eyes

Beyond the House of Commons, the public is not interested. There are no big demonstrations against our presence in Afghanistan as there were against the invasion of Iraq. Crowds occasionally turn out at Wootton Bassett in Wiltshire to honour and welcome home the bodies of dead British soldiers. The rest of us go on with our daily lives, eyes glazing over as the names of the fallen scroll across the ticker at the bottom of our television screens.

This pointless war was lost long ago. In early October 2008, Brigadier Mark Carleton-Smith, outgoing commander of UK forces in Afghan­istan, bluntly told a Sunday newspaper: "We're not going to win this war." He urged a "political settlement" with the Taliban. But we carried on fighting. On 15 October, 21-year-old Trooper James Munday from Birmingham was killed in an explosion in Garmsir. On 27 July 2009, the then foreign secretary, David Miliband, called for talks with the Taliban. Yet the war raged on. On the day of Miliband's speech, Trooper Phillip Lawrence, a 22-year-old from Birkenhead, was killed in an explosion in Lashkar Gah.

On 22 June this year, President Barack Obama announced the start of the US "drawdown" in Afghanistan, including the withdrawal of 33,000 troops by October 2012. On the same day, Foreign Secretary William Hague confirmed that, come 2015, British troops would not still be fighting in Afghanistan. However, in the meantime, we carry on fighting. And Hague has conceded that the British military's involvement will continue for "many years" after the end of combat operations in the country.

It cannot be said often enough: Britain's national security is not at stake in Afghanistan. None of the hundred or so Islamists imprisoned in the UK for terror-related offices hails from Helmand. Nor did our military presence in Afghanistan on 7 July 2005 prevent four suicide attacks on the London transport network that left 52 people dead.

As a US official noted at a recent briefing to reporters in Washington, DC ahead of the Obama speech: "We haven't seen a terrorist threat emanating from Afghanistan for the past seven or eight years." Even by the standards of the US government, the cynicism is breathtaking: play up the threat from terrorists to justify an intractable conflict until the time comes to start withdrawing and then reveal that the "threat" was, after all, non-existent.

Since 2001, 374 British service personnel have been killed in Afghanistan - a death toll that long ago exceeded the number of military casualties in the Falklands war (255) and the invasion of Iraq (179). The average age of British casualties in Afghanistan is 22; 28 of those 374 dead were teenagers.

Kids to slaughter

What have we done? Why have we sent our children to fight and die in a civil war between Pashtuns, Tajiks, Uzbeks and Hazaras? And why do we continue to do so? In May, it emerged that 16- and 17-year-olds, the youngest soldiers in the British army, were being taught basic counter-IED skills as part of their training at the Army Foundation College in Harrogate.

Whether we admit it or not, the war is lost. We are now negotiating with the Taliban from a position of weakness. It pains me to say this, but every British soldier who dies in Afghan­istan from this point onwards will have done so in vain. He or she will have died simply to preserve the tarnished credibility of the US and UK governments.

The parallels with Vietnam are unavoidable (and it is worth noting that the war against the Taliban has now dragged on longer than the one in Indochina). The US signed the Paris Peace Accords in January 1973; the Church-Case amendment officially ended US military involvement on 15 August 1973. Yet the last US soldier to be killed in Vietnam, 18-year-old Kelton Rena Turner, was gunned down on 15 May 1975. "How do you ask a man to be the last man to die for a mistake?" asked a young John Kerry, not long back from the front line in Vietnam, in testimony to Congress in 1971.

Before they enter the chamber for PMQs each Wednesday, Cameron and Miliband should ask themselves this: on what grounds do they prolong this futile conflict?