The Afghan war – a total waste. Unless...

Nato's Afghan campaign has not expired yet, but with Britain and those of her allies who have not already left now jostling for the 2014 exit, it is not too early to start the post-mortem. The first, book-length attempt to evaluate British expenditure of blood and treasure, by former frontline military intelligence officer Frank Ledwidge, should become a Defence Academy set text – if, that is, the generals can bear it, because it makes for very grim reading indeed. How much has this war cost us? And what, if anything, has it achieved?

More British soldiers have died in Afghanistan – 444 since 2001 – than in any other counterinsurgency campaign fought by Britain since the Boer War, which ended in 1902. More than 2,600 have been wounded, including 247 amputees. Other injuries, including future cases of post-traumatic stress disorder, are harder to enumerate, although they will likely surpass 5,000.

The financial costs are no less startling. Since 2006 we have spent at least £15m per day to maintain the British military presence in Helmand. By 2020, the UK will have spent, at a conservative estimate, £40bn in Afghanistan, 90 per cent of it on the military, equating to £650 for every resident of the UK, or more than £2,000 per tax-paying household. And for what? Successive prime ministers (but especially Gordon Brown) have told us that this war directly safeguarded Britain's streets; it was a necessary evil in which we "fought them over there so that we didn't have to fight them over here". But this was true only immediately after 9/11, when Bin Laden was still hiding on Afghan soil.

We have been fighting the Taliban this past decade, not al-Qa'ida, who were entirely driven out of Afghanistan by 2002. Our war was at least legal, in that it was mandated by the UN, but post-2001, was it really in Britain's long-term security interests? The Taliban – who, let it not be forgotten, publicly condemned the attacks of 9/11, of which they almost certainly had no prior knowledge – have never shown the slightest interest in attacking Western streets.

Instead, our investment in Afghanistan has allowed jihadists to portray us as infidel occupiers, a potent casus belli that has arguably made Britain's streets less safe than before – demonstrably so in the recent case of Michael Adebolajo, who allegedly linked the killing of Drummer Rigby in Woolwich to the presence of British troops in Muslim lands.

And what of the security of Afghans themselves, above all in Helmand, the focus of Britain's military effort? In February 2013, President Karzai declared that the situation had been better before the British first deployed there in 2006. Thousands of Helmandis have been internally displaced, an estimated 20,000 refugees from the province in Kabul alone. Thousands more have been accidentally killed in the fighting – many by us. Even the MoD, an institution not known for transparency in such matters, acknowledges that British taxpayers have, to date, compensated the families of 249 dead Helmandi civilians.

After eight years, UK forces have "stabilised" three of the 14 districts in Helmand – one of 34 Afghan provinces. "In terms of overall political significance," as Ledwidge puts it, "this might be the equivalent of three large market towns in Lincolnshire." Hundreds of millions have been spent on developing this backwater, a part of the vaunted state-building strategy known as "clear, hold, build". But this empty rhetoric and the contradictions inherent in a policy known as "hot stabilisation", are nowhere clearer than in our failure to eradicate Helmand's opium trade.

Poppies were the reason that Tony Blair opted to send British troops to Helmand in the first place – to be "lead nation" on counter-narcotics. But our troops were engaged in a broader Nato campaign to win hearts and minds – and that was never going to work if they went about destroying the farmers' livelihoods.

Today, Afghanistan produces about 85 per cent of the world's poppies, and 49 per cent of that is grown in Helmand – up from about 40 per cent in 2006. The irony is that the Taliban, when in power, did a better job of controlling poppy farming than we have done. They had good reason to do so, beyond the obvious point that drug use under Islam is forbidden. Afghanistan's war-torn society never used to have a drug problem, but it does now, with an estimated million opium or heroin addicts.

Some analysts think the real reason we have fought for so long in Helmand has been to support the special relationship – which was in need of some repair after our ignominious retreat from Basra in 2009. And yet, says Ledwidge, the "awful paradox" is that the reputation of our armed forces has not been restored in the eyes of the US, whose forces had to ride to our rescue in Helmand once again.

What will Helmand look like after we have gone? That depends on the willingness of the Kabul government to defend and build upon the progress so painfully won by our troops. The prognosis is poor. Ledwidge recounts how, in 2009, the then Foreign Secretary David Miliband asked two Afghan ministers how long they expected government authorities to stay on in Lashkar Gah, capital of Helmand, after Western forces left. "Twenty-four hours," they replied.

Our departure may well plunge the country back into civil war – in which case, all our blood and treasure will have been for nought. We have 18 months to broker the power-sharing agreement in Kabul that might prevent this. It is our only chance to leave Afghanistan in marginally better shape than we found it, with a chance of a more peaceful future. Talks with the Taliban, the prerequisite of any political settlement, have been official US and British policy for more than two years, but have repeatedly stalled. Frank Ledwidge's sombre review of the vast cost of our Afghan venture shows how vital it is that they are restarted – and that they succeed.

'Investment in Blood – The True Cost of Britain's Afghan War' by Frank Ledwidge (Yale UP)