Bribing leaders in the Afghan hinterland to take sides in the clash of civilisations is no easy task. The CIA used to offer local chiefs money for information, but then they would spend it so ostentatiously they effectively blew their own cover. Operatives used to give weapons before it turned out that the arms were being sold on and used against them.

But one day they decided that if they were going to rally the fence-sitters to the enlightenment cause they should strike below the belt. During a conversation with a chief in his 60s who had four younger wives, a CIA official pulled out a bag of Viagra. "Take one of these," he said. "You'll love it."

It was the beginning of a beautiful relationship. When the officer returned later in the week the once wary chief was all smiles and actionable intelligence. All he wanted in return was some little blue pills. "He came up to us beaming," the official told the Washington Post. "He said: 'You are a great man.' And after that we could do whatever we wanted in his area."

Another retired officer told the Post. "You didn't hand it out to younger guys, but it could be a silver bullet to make connections to the older ones." For elderly polygamous patriarchs, he explained, it could "put them back in an authoritative position".

But the line between authoritative and tyrannical in these matters in this part of the world is horrifically blurred. Recent legislation in Afghanistan, approved by the man the west put in power, in effect legalises rape in marriage. Other elements of the law make it illegal for a woman to leave home, seek work and education, or visit the doctor without her husband's permission – and she cannot refuse to "make herself up" or to "dress up" if her husband demands it. Under these circumstances handing out Viagra is a bit like bombing obscure targets from thousands of feet. The likelihood that anything good will come from it is highly remote.

It's a far cry from the days shortly before the war when Laura Bush took over her husband's weekly radio address to back the use of B-52s in the name of sex equality, and Cherie Booth railed against the Taliban from behind a burka. "For women to make a contribution they need opportunities, self-esteem and esteem in the eyes of their society," she argued. How you get from there to viagra and domestic despotism has been the warped story of the last eight years.

The grotesque contradictions between the political morality that underpins the bombing of Afghanistan and the realities of the occupation are stunning. This, lest we forget, was the respectable war. The just response. The war of necessity. The single most popular response to 9/11 – supported by 94% of Americans and 74% of Britons.

Today substantial majorities in both countries oppose it, and almost two-thirds of Americans think it is going badly – while more than half of Britons believe troops should never have been sent. It's not difficult to see why.

Just before the invasion Tony Blair delivered a series of rhetorical jabs at the charges of his critics, from the podium of the Labour party conference: "'Don't kill innocent people.' We are not the ones who waged war on the innocent. We seek the guilty. 'Look for a diplomatic solution.' There is no diplomacy with Bin Laden or the Taliban regime."

He made it sound not like war but a humanitarian mission with some strong-arm detective work thrown in.

But they have not found the guilty. And the innocent keep dying. Following the Nato air strike in Kunduz 10 days ago, bereaved civilians formed a line so that they might receive a charred corpse to call their own. "A man comes and says 'I lost my brother and cousin', so we gave him two bodies," said Omar Khan. "Another says I lost five relatives, so we gave him five bodies to take home and bury. When we had run out of bodies we started giving them limbs, legs, arms, torsos." Only five families left empty-handed.

Meanwhile, both the US and British military now concede that there can be no final settlement without involving the Taliban. In order to one day declare victory, the allies have to change the terms of what victory would mean almost every other day.

Failure in Afghanistan does not just represent a particular setback in a single war but the final unravelling of a grand narrative – the war on terror. These hostilities against an abstract noun were used not only to fight abroad but also to repress at home. In Britain the war on terror has not only curtailed our civil liberties but poisoned our racial discourse. Militarily, strategically, politically and diplomatically it has been an abject failure on its own terms. As a means of combating Islamism, facilitating democracy in the Middle East, integrating Muslims into the west and fighting terrorism it has proved not only ineffective but counterproductive. True, they have executed Saddam Hussein and toppled the Taliban (for now). But that is precious little to show for eight years of blood and treasure on this scale.

Both British and US governments understand this. The White House has abandoned the phrase "war on terror" in favour of "overseas contingency operations". The president's own counterterrorism adviser, John Brennan, last month explained that "describing our efforts as a 'global war' only plays into the warped narrative that al-Qaida propagates". David Miliband, Britain's foreign minister, now argues that terrorism "is a deadly tactic, not an institution or an ideology". General David Petraeus, the US commander, has said the western coalition in Iraq "could not kill its way out of the problems of insurgency and civil strife".

Indeed the only people who believe the strategy is still viable are the neocons, and the "muscular liberals" who attempted to give them cover from the left. The former's bid for America to unilaterally impose its will on the rest of the world has foundered on military and diplomatic realities.

The latter's effort to realign the left in support of American imperialism and in fear of political Islam was always a stretch and has proved central to New Labour's undoing. Fancying themselves as Orwell during the 30s, a raft of British commentators claimed not to be leaving the left but returning it to its true principles. They cast their critics as a blend of the querulous Neville Chamberlain and craven apologists for Stalin. And having created all these straw men, they then proceeded to waterboard them. Those who didn't support war abroad and assimilation and state repression at home were branded as soft on Islam, women's rights, gay rights, forced marriage and dictatorship.

The vehemence with which both sides defend their positions is in direct proportion to the degree to which their positions have been utterly discredited.

But while the intellectual foundations of the war on terror have been discarded, the global infrastructure erected to sustain it – rendition, secret prisons, Guantánamo, the Patriot Act, wire-tapping – still stands.

Barack Obama has increased troop levels in Afghanistan and delayed Iraq withdrawal. The language that has dominated the last eight years has changed – but the logic persists. Having retired the term "war on terror", he must retire the war itself.