The US did not put enough pressure on former Afghan president Ashraf Ghani to share power with the Taliban, said Zalmay Khalilzad, who recently resigned as US envoy to Afghanistan.
Mr Khalilzad, speaking for the first time since his resignation was announced on October 18, also told CBS on Sunday of his reservations about the decision by the Biden administration to lift conditions on the withdrawal deal he had negotiated with the Taliban during the administration of president Donald Trump.
The agreement signed on February 29, 2020, between Washington and the Taliban, which excluded Mr Ghani's government in Kabul, paved the way for the US to end its longest war.
But it was "a conditions-based package" that included negotiations between the insurgents and Kabul, as well as a permanent, comprehensive ceasefire, said Mr Khalilzad, who was born in Afghanistan.
But once in the White House, President Joe Biden decided "to do a calendar-based withdrawal" without regard to those conditions, he said.
"That was a decision made way above my pay grade," Mr Khalilzad said.
Talks between the insurgents and Kabul had begun but were dragging, and Washington feared the Taliban would resume attacks on US forces if they stayed in the country much longer – a situation Mr Khalilzad acknowledged as he admitted things did not work out the way he had wanted.
He placed most of the blame on Mr Ghani, who he said never agreed to share power with the Taliban.
"They preferred the status quo to a political settlement," he said of the government.
"And then when it became clear that the US was leaving, then they miscalculated the effects of the continuing war. They were not serious about the political settlement.
"It's my judgment that we didn't press him hard enough. We were gentle with president Ghani. We used diplomacy. We encouraged him."
Mr Khalilzad said that under the original conditional withdrawal agreement, the Taliban would have eventually agreed to sharing power, although his evidence for that was unclear.
Mr Biden had set a departure date of August 31 for the final withdrawal.
But in the months and weeks leading up to that date the Taliban offensive surged. On August 15, Mr Ghani fled Kabul as government authority crumbled and the Taliban marched into the capital city.
Mr Khalilzad – derided in Afghanistan for, among other things, cutting Kabul out of the US-Taliban deal, and who has also been much criticised in Washington since the takeover – has blamed Mr Ghani before.
He told the Financial Times in September that Mr Ghani's abrupt exit scuttled a deal in which the Taliban would delay entering Kabul and negotiate a political transition.
Mr Ghani has apologised for how his government ended but said he left on the advice of palace security to avoid bloody street fighting.
The Taliban had been demanding his resignation as part of any transitional government.
Ultimately, they named a caretaker government that has no non-Taliban or women, and that includes US-designated terrorists.