General Sir Nick Carter said the collapse of the Afghan government and the Kabul evacuation in August was ‘shocking’.
He said it was “probably still true” that the majority of Afghans would not support the Taliban regime.
Asked whether he had been too positive about the Afghan government’s ability to resist the Taliban, Gen Carter – who was chief of the defence staff until the end of November – said he had needed to be upbeat in his public statements to avoid undermining Ashraf Ghani’s administration.
Gen Carter defended using former Afghan president Hamid Karzai’s description of the Taliban as “country boys” who lived by a “code of honour”.
He told BBC Radio 4’s Today programme: “I think at the end of the day it comes down to politics, the local politics in Afghanistan, and I don’t think there was ever a true understanding of the political dynamics on the ground and therefore the ability to be able to rationalise that.”
The speed of the collapse resulted in a hasty evacuation from Kabul after 20 years of UK involvement, with people desperately trying to escape the Taliban advance.
“They were shocking moments for all of us,” Gen Carter said.
“And of course you think so many people in times like that – you think of all of those who’ve committed their lives to the cause over the last 20 years, you think of the Afghan people, and of course you wonder how it ended like that.
“Because we thought we had a good plan, we we’re going to deliver something at the end of all of that. And so it was some it was a true shock, I think, to most of us, who’d been involved.”
He said the West now needed to engage with the Taliban to help get aid to the Afghan population that was facing a humanitarian crisis.
“I also think it’s important that we engage because we might be able to encourage them to govern in a different way. And the fact that they are governing in an utterly uninclusive way, is something that we need to worry about,” he said.
On an edition of Today guest-edited by Gen Carter, former Afghan president Ghani defended his decision to flee the country.
Mr Ghani told Gen Carter that when he woke up on August 15 he had “no inkling” it would be his last day in Afghanistan.
He said: “I did not know where we were going, only when we took off did it become clear that we were leaving. So this really was sudden.”
He said what happened was “a violent coup, not a political agreement, or a political process where the people have been involved”.