Biden makes his case on U.S. withdrawal from Afghanistan as the Taliban gain ground. Video

President Biden made an impassioned case on Thursday that the United States could no longer afford the human cost or strategic distraction of fighting the war in Afghanistan, arguing that the United States had achieved its initial objective — routing Al Qaeda from the country and hunting down Osama bin Laden — and that Afghanistan’s government and forces must be responsible for their own future.

In a half-hour long talk, Mr. Biden was by turns defensive about his decision to leave the country and angry at his critics, at one point asking whether they would send their sons and daughters to fight in what has devolved into a civil war. He said all American combat troops would be out of the country by Aug. 31, and made the case that the American drawdown “is proceeding in a secure and orderly way prioritizing the safety of our troops as they depart.”

“We did not go to Afghanistan to nation-build,” Mr. Biden said from the East Room of the White House. “And it’s the right and the responsibility of Afghan people alone to decide their future and how they want to run their country.”

Mr. Biden’s speech, just days after the U.S. pulled out of Bagram Air Base, which was the operations center of the 20-year war, marked his formal acknowledgment that the U.S. could not alter the country’s course.

“No nation has ever unified Afghanistan, no nation. Empires have gone there and not done it,’’ he said in a reference to the British occupation of the country in the 19th century and the Soviet effort to gain control three decades ago. Both efforts failed, and Mr. Biden was, in essence, adding the United States to the list.

“I will not send another generation of Americans to war in Afghanistan with no reasonable expectation of achieving a different outcome,’’ Mr. Biden said. But in answering questions from reporters after his prepared remarked, he also rejected the contention that American intelligence had concluded that Kabul, the capital, could fall, and said it would not be comparable to what happened in Saigon in 1975, when the North Vietnamese took over the south.

“There is going to be no circumstance in which you are going to see people being lifted off the roof of an embassy of the United States from Afghanistan,’’ Mr. Biden contended. At another point he added: “I trust the capacity of the Afghan military.”

The departure of American troops has been accompanied by reports of increasingly dire situations on the ground as local government forces crumble before the Taliban, which is gaining territory as it nears Kabul. In the span of just over two months, the Taliban have managed to seize at least 150 of Afghanistan’s 421 districts.

Mr. Biden seemed to acknowledge that reality when he said to reporters, “The mission hasn’t failed — yet.”

In an interview on Fox News on Wednesday, John Kirby, the Pentagon spokesman, said that the Biden administration continued to “push for a negotiated peaceful, political settlement,” while adding that the Pentagon still had the ability to support Afghan forces even without a formal presence in the country.

“We are all concerned about the security situation the ground,” Mr. Kirby said. “There’s no question about that.”

Only 650 troops are expected to remain in the country to protect the U.S. Embassy in Kabul and Kabul International Airport. But officials have said that the Pentagon would be authorized, at least through September, to move 300 additional troops into Afghanistan if needed for security or emergencies.

Asked to address concerns that Taliban control could lead to more widespread impediments to Afghan women pursuing education, Mr. Biden recalled a trip in which a female student told him that without an American military presence, she would be unable to pursue her dream of becoming a doctor.

“The young woman said ‘you can’t leave, you can’t leave’ — it was heartbreaking,” Mr. Biden said. “‘You can’t leave,’ she said, ‘I want to be a doctor.’”

But Mr. Biden went on to insist that even with U.S. forces withdrawing, efforts to keep women safe would be ongoing.

“Well, that’s why we spent so much time and money training the Afghan security forces, to do the work of defending them,” he said.

As the Taliban consolidates power in Afghanistan, the threat to American allies who remain in the country grows. Senior officials have said that the Biden administration is preparing to relocate thousands of Afghan interpreters, drivers and others who worked with American forces to other countries in an effort to keep them safe while they apply for entry to the United States.

In his remarks, Mr. Biden also touched on plans for humanitarian assistance to Afghanistan, an issue that came up last week when Afghanistan’s president, Ashraf Ghani, visited the White House. The Biden administration has committed $266 million in humanitarian aid and $3.3 billion in security assistance.

“I stressed in my meeting just two weeks ago with President Ghani and Chairman Abdullah, Afghan leaders have to come together and drive toward a future that the Afghan people want and they deserve,” Mr. Biden said in his remarks.