A week later, on July 2, Mr. Biden, in an ebullient mood, gathered a small group of reporters to celebrate new jobs numbers that he said showed that his economic recovery plan was working. But all the questions he received were about news from Afghanistan that the United States had abandoned Bagram Air Base, with little to no notice to the Afghans.

“It’s a rational drawdown with our allies,” he insisted, “so there’s nothing unusual about it.”

But as the questions persisted, on Afghanistan rather than the economy, he grew visibly annoyed. He recalled Mr. Ghani’s visit and said, “I think they have the capacity to be able to sustain the government,” though he added that there would have to be negotiations with the Taliban.

Then, for the first time, he was pressed on what the administration would do to save Kabul if it came under direct attack. “I want to talk about happy things, man,” he said. He insisted there was a plan.

“We have worked out an over-the-horizon capacity,” he said, meaning the administration had contingency plans should things go badly. “But the Afghans are going to have to be able to do it themselves with the Air Force they have, which we’re helping them maintain,” he said. But by then, most of the U.S. contractors who helped keep the Afghan planes flying had been withdrawn from Bagram along with the troops. Military and intelligence officials acknowledge they were worried that the Afghans would not be able to stay in the air.

By July 8, nearly all American forces were out of Afghanistan as the Taliban continued their surge across the country. In a speech that day from the White House defending his decision to leave, Mr. Biden was in a bind trying to express skepticism about the abilities of the Afghan forces while being careful not to undermine their government. Afterward, he angrily responded to a reporter’s comparison to Vietnam by insisting that “there’s going to be no circumstance where you see people being lifted off the roof of an embassy of the United States from Afghanistan. It is not at all comparable.”

But five days later, nearly two dozen American diplomats, all in the Kabul embassy, sent a memo directly to Mr. Blinken through the State Department’s “dissent” channel. The cable, first reported by The Wall Street Journal, urged that evacuation flights for Afghans begin in two weeks and that the administration move faster to register them for visas.

The next day, in a move already underway, the White House named a stepped-up effort “Operation Allies Refuge.”