“We’ve seen this movie before,” Mr. Austin told his boss, in a reference to the Obama-era withdrawal from Iraq, which was followed by the rise of the Islamic State. The United States ended up returning to Iraq and launching five years of airstrikes in Iraq and Syria to help Iraqi security forces beat back that insurgent group.
Mr. Biden has argued for pulling out of Afghanistan for years. In 2009, while serving as vice president, he argued for a minimal force, only to be overruled as Mr. Obama ordered a surge of forces, then a rapid drawdown.
But a dozen years later, as president, he made the decision to withdraw, one of the most significant decisions of his presidency so far. And despite the likelihood that the White House will confront terrible images of human suffering and loss in the coming weeks and months, Mr. Biden has vowed to press ahead regardless of the conditions on the ground.
Polls show that large numbers of Americans in both parties support leaving Afghanistan.
Mr. Biden, declaring that the United States had long ago accomplished its mission of denying terrorists a haven in Afghanistan, said in April that all American troops would leave the country by Sept. 11. That date has since been moved up to Aug. 31, giving the Pentagon — and Afghan forces — just over a month to slow the Taliban surge.
Administration and military officials have voiced conflicting views on whether the United States will continue airstrikes after Aug. 31 to prevent Afghan cities and the Afghan government, led by President Ashraf Ghani, from falling. But even if the airstrikes continue, they can only do so much; the bulk of the effort will have to come from Afghan forces on the ground.
In any event, Kunduz was never going to be the Afghan city that might prompt Mr. Biden to rethink his strategy, two U.S. officials said on Sunday on condition of anonymity.
His hand might be forced if Taliban forces are on the verge of overrunning Kandahar, Afghanistan’s second-largest city, or even Kabul, where the United States maintains an embassy with some 4,000 people.
Helene Cooper and Katie Rogers reported from Washington, and Thomas Gibbons-Neff from Kabul. Eric Schmitt contributed reporting from Washington.