The Taliban's startling advances in Afghanistan threaten to be a stain on US President Joe Biden's record, but he has stood firm on withdrawing troops and believes the public is with him.

Twenty years of investment that cost $2 trillion and nearly 2,500 US lives were disintegrating within days as the Taliban seized two of the largest cities with little resistance and closed in on the capital Kabul.

Republican rivals predictably attacked Biden but he also faced the most critical coverage of his presidency, with television networks juxtaposing images of Afghanistan's collapse with his remarks a little more than a month ago that “the Taliban overrunning everything and owning the whole country is highly unlikely".

In a scathing editorial, The Washington Post said that Biden had put at risk the real progress in Afghanistan since 2001 including education for girls, banned by the Taliban when they last ruled.

“Afghan lives ruined or lost will belong to Mr. Biden's legacy just as surely as any US dollars and lives his decision may save,” the newspaper wrote.

The United States was rushing back 3,000 troops — roughly the same number removed in this month's final withdrawal — to evacuate embassy staff and was flying out Afghans whose work with US forces puts them at risk.

But Biden, who through his decades in public life earned a reputation for empathy, has been unmoved when asked about Afghan losses and instead speaks of protecting US troops, a deeply personal matter as his late son Beau served in Iraq.

Both the former vice president and US opinion polls have shared his view for years.

VoteVets, an advocacy group, hailed Biden for finally “having the strength to stand up to those who want endless war.”

Lack of planning?

Biden argues that the United States long ago achieved its main goal of defeating Al-Qaeda after the September 11, 2001 attacks and had done more than enough by training 300,000 Afghan troops.

“They've got to fight for themselves, fight for their nation,” Biden said on Tuesday.

Administration officials say that delaying the pullout was only prolonging the inevitable.

But Andrew Wilder, an Afghanistan expert who visited in June, said the administration could have devoted more time to preparing for the expected effects and that it was not “an orderly and responsible withdrawal”.

“I think it's hard not to conclude that, not the US withdrawal, but the way in which we withdrew had a critical role to play in this,” said Wilder, vice president for Asia studies at the US Institute of Peace.

The US pullout also created “an air of inevitability” that sapped the Afghan will to fight, even if the Taliban remain unpopular.

“To me the psychological factor is what we didn't adequately factor in,” Wilder said.

In pictures: The human cost of the Taliban's gains

Top Senate Republican Mitch McConnell said Biden allowed a “massive, predictable and preventable disaster” and former president Donald Trump issued a statement denouncing the “tragic mess” and writing in all caps, “Do you miss me yet?” But Trump himself set in motion the withdrawal with a February 2020 deal with the Taliban.

Critics have drawn parallels to the chaotic fall of Saigon in 1975 but the US president at the time, Gerald Ford, had been in office for less than a year and is rarely cast by historians as the sole to blame for the tortured US experience in Vietnam.

US apathy

Biden had cast Afghanistan as a costly side issue when the United States needs to focus on a larger challenge from China.

Brian Katulis, a senior fellow at the left-leaning Center for American Progress, wondered how Taliban gains would affect Biden's stated mission of defending democracy in the face of authoritarians.

But Katulis said it was unclear how much of a political price Biden would pay. The US withdrawal from Iraq in 2011, pushed by Biden, was popular until the rise of the Islamic State extremist movement.

“This really depends on how ugly it gets,” Katulis said.

“If it's a series of atrocities just involving Afghans, you can look to Syria as an example where there is just a global shrug of indifference of saying there's nothing we can do,” he said.

“But if Americans are involved, then all bets are off. “