The West should have tried talking to the Taliban a decade ago, the UK's top general in Afghanistan has said.
Gen Nick Carter told the Guardian it would have been easier to find a political solution when they were on the run in 2002.
Prime Minister David Cameron acknowledged that the original settlement for Afghanistan "could have been better arranged".
His comments come days after planned negotiations with the Taliban stalled.
Gen Carter also warned Afghan forces would need military and financial support after troops leave in 2014.
The Kabul government would have only shaky control over some areas, he said.
A major conference on the future of Afghanistan held in Bonn, Germany, over a decade ago did not include the defeated Taliban former government of Afghanistan.
Gen Carter, deputy commander of the Nato-led coalition, acknowledged it was easy to be wise with the benefit of hindsight but added: "Back in 2002, the Taliban were on the run.
"I think that at that stage, if we had been very prescient, we might have spotted that a final political solution to what started in 2001, from our perspective, would have involved getting all Afghans to sit at the table and talk about their future.
"The problems that we have been encountering over the period since then are essentially political problems, and political problems are only ever solved by people talking to each other."
Speaking as he visited UK troops in Camp Bastion on Armed Forces Day, The prime minister said he was encouraged that the Taliban no longer wanted Afghanistan to be "a haven for terror".
He said: "You can argue that the settlement we put in place in 2001 could have been better arranged.
He added: "You have to remember why we came here and that was because the Taliban regime allowed Al Qaeda to have a base in Afghanistan, so that's why that regime was removed, why an Afghan democracy has been created and why we have now built up an Afghan National Army and police force which are capable of securing this country.
"But do we want people to give up weapons to give up an armed struggle and join a political process so that everyone in Afghanistan can be part of that political future, yes."
Doha talks row
Defence Secretary Philip Hammond said it would have been "very difficult" to negotiate with the Taliban a decade ago.
He said: "I suspect ten years ago it would have been very difficult.
"We've reset the parameters of the debate by building the Afghan security forces, by supporting the Afghan government to reach out across the country, delivering services to the people in a way that has given it legitimacy, and I think the time is right now for that negotiation to take place."
Last week, US Secretary of State John Kerry expressed caution over whether peace talks on Afghanistan with the Taliban could take place.
A row over the status of a Taliban office in Qatar's capital Doha has overshadowed efforts to start peace negotiations there.
BBC defence correspondent Caroline Wyatt said the row had simply underlined the diplomatic and practical difficulties that remained for anyone wishing to talk to the Taliban.
Gen Carter said he was confident that Nato's handover of security to Afghan forces would eventually bring the Taliban to the negotiating table.
He said that overall the police and army had been shaped into sustainable institutions strong enough to protect a critical presidential election next year and guarantee stability for the majority of the country after Western forces withdrew.
However, he added that the Afghan army and police would still need help in the years to come because they had been built up very quickly.
However, he expressed optimism about Afghanistan's future as long as the US and its allies came through on promises of financial and military support.
Some 8,000 British troops are still serving in Afghanistan, around half of them at Camp Bastion in Helmand province, many of them still mentoring or advising Afghan forces.
Until last year, the UK had 137 bases in Helmand but the gradual withdrawal ahead of the end of combat operations by 2015 means the mission is gradually changing with just 13 bases still operating.