Originally Published: 08 JUL 21 05:07 ET

Updated: 08 JUL 21 17:11 ET

By Kevin Liptak, CNN

(CNN) -- President Joe Biden delivered a vehement defense Thursday of his decision to end the war in Afghanistan, insisting no amount of sustained American presence there could resolve the country's own intractable problems.

Instead, Biden argued American resources -- lives, money and attention -- were better focused elsewhere. Pressed on the likelihood of a total Taliban takeover, Biden grew defensive and said nothing in Afghanistan's history suggested there was anything the US could do about it.

Now, he said, it is up to the Afghans to defend their country, determining there was nothing more the United States could do after two decades of war.

"I will not send another generation of Americans to war in Afghanistan with no reasonable expectation of achieving a different outcome," Biden said from the East Room after receiving an update from his national security team on the deteriorating security situation on the ground.

Thursday's remarks were the first time Biden discussed his decision to end the Afghanistan war in a formal setting since announcing the withdrawal plans in April. A longtime skeptic of the war, Biden resisted pressure from military leaders to maintain a troop presence, determining instead there was nothing that could be accomplished by remaining.

The hasty draw-down has coincided with major Taliban gains and a growing fear that without American military support, the Afghan government will collapse.

Biden has not second guessed himself, even as his decision comes under scrutiny from members of Congress and others who argue the US must do more to prevent a full-blown civil war.

He rejected comparisons on Thursday to the end of the Vietnam War, saying there was "no circumstance in which you are going to see people being lifted off the roof of an embassy of the United States from Afghanistan."

Instead, he said the pace with which the Taliban have gained ground in recent weeks has only solidified the merits of his decision.

"The current security situation only confirms that just one more year fighting in Afghanistan is not a solution, but a recipe for being there indefinitely," Biden said on Thursday.

In his remarks, Biden said American troops will be done with the military mission by August 31 -- weeks ahead of his original schedule. After dismantling al Qaeda in the country and killing Osama bin Laden, Biden said the military mission had been accomplished.

"We achieved those objectives, that's why we went. We did not go to Afghanistan to nation-build, and it's the right and responsibility of the Afghan people alone to decide their future and how they want to run their country," Biden said.

The withdrawal is now more than 90% complete, officials said this week. The last US troops left Bagram Air Base last week, marking the end of the American presence at the sprawling compound that became the center of military power in Afghanistan.

Some Afghan soldiers told CNN they only found out the Americans were leaving that very day. And Afghan officials accompanying CNN on a tour of Bagram on Monday confessed they were only then getting access to much of the base and working out what had been left behind.

One senior officer said he was notified last Thursday that his forces had less than 24 hours to secure the perimeter of the base.

US intelligence services, military commanders and members of Congress have all warned that the Afghan government won't be able to stand up to the Taliban without the backing of American firepower. The Taliban are already moving rapidly to take over districts in the northern parts of Afghanistan, leading US military commanders to raise the prospect of a civil war once US troops are gone.

Biden offered a less pessimistic view on Thursday, saying he did not believe it was inevitable the Taliban would take control of the country.

But he said discord was embedded in Afghanistan in ways impossible for the US to resolve.

"No nation has ever unified Afghanistan, no nation. Empires have gone there and not done it,'' he said.

Now, he said, the US must move on.

"The United States cannot afford to remain tethered to policies creating a response to a world as it was 20 years ago. We need to meet the threats where they are today," he said.

Facing the prospect of a Taliban takeover within a year -- accompanied, most likely, by devastation and suffering -- Biden aimed to convey continued support for the country.

He directly addressed the thousands of Afghans who assisted the United States during the war who are now waiting on their visa applications, vowing to allow them to come to the United States.

"Our message to those men and women is clear: There is a home for you in the United States, if you so choose and we will stand with you, as you stood with us," Biden said.

The visa process has been slow for those looking to leave Afghanistan, including translators and other workers, leading officials to develop plans to relocate them to a third country.

The President's aides have said the relocation for those waiting on visas will be finished before the drawdown is complete at the end of August, without specifying how many people they plan to move or confirming where they will go while they wait.

One administration official told CNN the first flights for the interpreters could begin in two weeks.

Other big decisions have yet to be made, including whether and how the US will use drones in the future to target suspected terrorists in Afghanistan and how to secure the civilian airport in Kabul.

Last week, Biden bristled under questioning on Afghanistan, saying he'd rather discuss the matter after the July 4 holiday.

And before he spoke on Thursday, his press secretary said ending America's longest war was not a moment for celebration.

"We're not going to have a 'mission accomplished' moment in this regard," Jen Psaki said. "It's a 20-year war that has not been won militarily."

This story has been updated with additional developments Thursday.