On 1 October 2001, three weeks after the 9/11 attacks and six days before the bombing of Afghanistan began, there was a small protest march in Washington.

The marchers wore badges saying “Don’t Turn Tragedy into War” and “Our Grief Is Not a Cry for War”, and argued that war was not the inevitable response to the terrorist outrage.

The protest fell on the deafest of ears in the wake of the atrocities committed by al-Qaida. The headline on the New York Times report on the march was “Marchers Oppose Waging War Against Terrorists”.

Twenty years on, in the wake of a stunning defeat for the US and its allies and the return of the Taliban to power, the questions the marchers were asking are being asked again – this time in a spirit of resignation and despair.

Amid the shock of Kabul’s fall, the chaos surrounding evacuation efforts, and the pervasive sense of betrayal, there is widespread fear that the past 20 years, the tens of thousands of lives lost and the $2tn spent, may all have been for naught.

“To be very honest at the moment, I’m losing everything that we have worked so hard for, everyone in my family, everyone in my tribe, everyone in my district, including the whole province,” Pashtana Durrani, a campaigner for girls’ education in Kandahar, told the BBC. “We have to flee, we have to abandon the houses that we have worked hard for, and to give up all those sacrifices that we made.”

Lt Col James Cho, a former US air force intelligence officer now a defence council member on the Truman National Security Project, said: “I’m torn, sad and angry, but I’m all these things because I bore witness to the untold sacrifices of amazing men and women, and now I struggle with whether any of it mattered.”

“To be honest, the more I thought about whether it was all worth it for some grander strategic vision or goal, I think, the more I just kind of despair,” Cho said. “What I settled on was that I went there because my brothers and sisters in arms were also going there and making sure that we were there taking care of each other.”

The original war aim of the US and its coalition partners was to prevent Afghanistan being a launching pad for al-Qaida attacks on the west. By that limited benchmark, the military presence has been successful, but it is uncertain whether that success will now be reversed.

“The Taliban-al-Qaida relationship is as firm as it’s ever been,” Charles Lister of the Middle East Institute argued. “The Taliban’s political positioning may have evolved somewhat over the years, but relationships like that are far more resilient.”

The US special inspector general for Afghanistan reconstruction (Sigar) issued a report on Tuesday on the broader balance sheet for the military intervention in Afghanistan, and it was similarly damning.

“If the goal was to rebuild and leave behind a country that could sustain itself and pose little threat to US national security interests, the overall picture in Afghanistan is bleak,” the report said.

It recognised all the gains in life expectancy, child mortality and literacy but added: “Despite these gains, the key question is whether they were commensurate with the US investment or sustainable after a US drawdown. In Sigar’s analysis, they were neither.”

Alongside the initial dire accounting, however, there are the beginnings of a reckoning of what can be salvaged. The Afghanistan the Taliban is inheriting now is very different from the Afghanistan of 2001.

Over the 20 years, infant mortality rates fell by half. Hardly any girls went to school under the first Taliban regime. Now, more than one in three teenage girls can read and write. In 2005, fewer than one in four Afghans had access to electricity. Now, almost all do.

These are gains that are difficult to erase, and it would be self-destructive for the Taliban to try. In that sense, though the military defeat is crushing and unambiguous, the degree of success or failure of the sacrifices of the intervening two decades is a battle still to be fought.

“It’s very easy to look at this situation and think of it as we just lost the Super Bowl and the game is over,” said Dominic Tierney, political science professor at Swarthmore College and author of a new book: The Right Way to Lose a War: America in an Age of Unwinnable Conflicts. “What actually happens with these complex modern wars is that they don’t really have a clear endpoint.

“Right now is an absolutely critical moment, what could be decided is: do we have a kind of manageable loss, or a total catastrophe. And what people need to realize is the gap between those two things is huge,” Tierney said.

He argues that, alongside the evacuation of as many refugees as possible, the US should start using all means at its disposal, including partnership with China, Russia and other world powers who will have a stake in Afghanistan, to try to mitigate the scale of defeat.

“The victory of the Taliban is going to create a lot of friction between the Taliban and a lot of regional actors. And if the United States is savvy, it might be able to actually take advantage of that,” Tierney argued.

For the Afghans left behind by the hasty retreat of the US, UK and other western powers, mitigating defeat will be a matter of unarmed resistance, a refusal to give up what they have gained for themselves.

Fatima Ayub, an Afghan now working as Washington director of Crisis Action, wrote on Twitter: “I know this for certain: if the Taliban insist on stripping joy from Afghans, the most widely traumatized and forsaken people on Earth, they will end their own rule.”