In a final blow of the 20-year war, U.S. envoys close their embassy and exit Kabul.

WASHINGTON — American diplomats have left Afghanistan, and the U.S. Embassy in Kabul will remain closed, Secretary of State Antony J. Blinken said on Monday, after the military announced that it had completed its withdrawal from the country.

The disintegration of diplomacy was a stunning turnabout from plans to stay and help Afghanistan transition from 20 years of war and to work toward peace, however tenuous, with a government that would share power with the Taliban. This month, Mr. Blinken had pledged that the United States would remain “deeply engaged” in Afghanistan long after the military left.

But with the Taliban firmly in control, what was one of the largest U.S. diplomatic missions in the world will for now be greatly scaled back, based in Doha, the Qatari capital, and focused largely on processing visas for refugees and other immigrants.

“Given the uncertain security environment and political situation in Afghanistan, it was the prudent step to take,” Mr. Blinken said in remarks at the State Department.

He sought to portray the departure as a “new chapter of America’s engagement with Afghanistan.”

“It’s one in which we will lead with our diplomacy,” Mr. Blinken said, commending the U.S. diplomats, troops and other personnel who had worked at the embassy, which just last month had employed around 4,000 people — including 1,400 Americans.

Left uncertain was whether American efforts to stabilize the Afghan government would continue — the main thrust of years of painstaking work and negotiations with leaders in Kabul that were supported by billions of dollars in American taxpayer funding.

Instead, Mr. Blinken said that any engagement with the Taliban — a longtime U.S. enemy that seized power when President Ashraf Ghani fled Afghanistan on Aug. 15 — “will be driven by one thing only: our vital national interests.”

Exactly four weeks earlier, on Aug. 2, Mr. Blinken had left little doubt that the Biden administration intended to keep the U.S. Embassy in Kabul open.

“Our partnership with the people of Afghanistan will endure long after our service members have departed,” he said then. “We will keep engaging intensely in diplomacy to advance negotiations between the Afghan government and the Taliban with the goal of a political solution, which we believe is the only path to lasting peace.”

As many as 200 American citizens, and tens of thousands of Afghans, were left behind in a two-week military airlift that Mr. Blinken called one of the largest evacuation efforts in U.S. history. He demanded that the Taliban keep its word and allow them to leave safely once they had exit documents in hand.

More than 123,000 people were evacuated from Kabul in recent weeks, including about 6,000 Americans.

Mr. Blinken also said that the United States would closely watch the Taliban’s efforts to stanch terrorism in Afghanistan, as the group has said it will do, and would continue to work with the international community to provide humanitarian aid to millions of Afghans who need food, medicine and health care after decades of war and political instability.

He struck a resolute tone about the diplomatic retreat, and in reminding Americans about the cost of the conflict.

America’s longest war, with its casualties and the resources that were sunk into it over the past 20 years, “demands reflection,” Mr. Blinken said.

“We must learn its lessons, and allow those lessons to shape how we think about fundamental questions of national security and foreign policy,” he said. “We owe that to future diplomats, policymakers, military leaders, service members. We owe that to the American people.”