NEW YORK -- U.S. President Joe Biden on Thursday defended his decision to withdraw from Afghanistan as the Taliban make rapid gains against the country's faltering military, which the U.S. spent billions to recruit, train and arm.

"The United States cannot afford to remain tethered to policies created in response to a world as it was 20 years ago," said Biden at a briefing in the White House, doubling down on his decision to end America's longest war. "We need to meet the threats where they are today. Today the terrorist threats have metastasized beyond Afghanistan."

"We also need to focus on shoring up America's core strengths to meet the strategic competition with China and other nations," said Biden, explaining his administration's decision to concentrate on Beijing and the Indo-Pacific region. "That is really going to determine our future."

"We will be more formidable to our adversaries and competitors over the long run if we fight the battles of the next 20 years, not the last 20 years," he said, adding that fighting COVID-19, preparing for the next pandemic, tackling the "existential threat of climate change," and creating "international norms for cyberspace and the use of emerging technologies" were the future threats that America must concentrate on.

"Speed is safety," said Biden about the withdrawal, announcing that the U.S. mission will conclude even earlier than expected, by Aug. 31.

Biden had previously announced the symbolic date of Sept. 11 to withdraw all U.S. military personnel from the country.

No U.S. forces have been lost in the withdrawal so far, said Biden.

"There was never any doubt" that the military would withdraw efficiently and professionally, he noted.

But the U.S. president has faced questions about the future of Afghanistan, which members of the country's military and intelligence community have warned is headed into imminent civil war.

Taliban co-founder Mullah Abdul Ghani Baradar, center, arrives with other members of the Taliban delegation for an international peace conference in Moscow, Russia in March.    © AP

The mission in Afghanistan was "to deliver justice to Osama bin Laden and to degrade the terrorist threat. ...We achieved those objectives. That's why we went. We did not go to Afghanistan to nation-build," said Biden.

"We went for two reasons. One was to bring Osama bin Laden to the gates of hell," said Biden. "The second reason was to eliminate al-Qaida's capacity to deal with more attacks on the United States from that territory. We accomplished both of those objectives. Period. ...That job had been over for some time."

"No nation has ever unified Afghanistan," said the president. "Empires have gone in there and not done it."

"The only way to peace in Afghanistan is that they [the government] work out a modus vivendi with the Taliban," said Biden, hinting at his administration's preference for a power-sharing arrangement in the country.

"The likelihood that there is going to be one, unified government in all of Afghanistan controlling the whole country is highly unlikely."

"There is no 'mission accomplished,'" said Biden in reference to former President George W. Bush's infamously premature declaration of victory in Iraq in 2003.

"It's up to the Afghan people alone to decide their future and how they want to run their country," he said, placing the onus on the Afghan security forces during much of the remarks.

"We have trained and equipped 300,000 current serving members of the Afghan military and national security force," he said. "We provided them all the tools and training equipment of any modern military. ...They clearly have the capacity to sustain the government in place. The question is, will they generate the kind of cohesion to do it."

Responding to a question, Biden said that a Taliban takeover of Afghanistan was "not inevitable," saying the Afghan military was as "well equipped as any army in the world, against something like 75,000 Taliban."

He also reacted angrily to a question about trusting the Taliban.

"That's a silly question. Do I trust the Taliban? No. But I trust the capacity of the Afghan military, who are better trained, better equipped and more competent in terms of conducting war."

"But Afghan leaders have to come together and drive towards the future," warned Biden several times over the course of his speech, pledging civilian and humanitarian assistance and said his administration would continue speaking for women and girls.

Biden also pledged to pursue a course of "determined diplomacy" to "end this senseless violence."

"To be clear, countries in the region have an essential role to play in supporting a peaceful settlement," said Biden, referring to neighboring countries like Pakistan, Iran, China and Russia, all of which have been part of the ongoing peace process which has been faltering with the U.S. withdrawal. "We will work with them, and they should help step up their efforts as well."

Biden also pledged to protect the thousands of translators and interpreters who have helped the U.S. military, saying 2,500 special immigrant visas have been issued to these personnel who helped the U.S. military. He said a relocation flights and U.S. facilities outside the continental U.S. have also been set up.

"There is a home for you in the United States, if you so choose. We will stand with you, just as you stood with us," said Biden.

Biden said that he made the decision to withdraw from Afghanistan "with clear eyes."

"Nearly 20 years of experience has shown us ... that one more year of fighting in Afghanistan is not a solution, but a recipe for being there indefinitely."

"We are repositioning our resources and adapting our counterterrorism posture to meet the threats where they are now: significantly higher in South Asia, the Middle East and Africa," said Biden, reiterating that any threat emanating from Afghanistan will be dealt with.

"We are developing an over-the-horizon counterterrorism capability that will allow us to keep our eyes firmly fixed on any direct threats to the United States from the region and allow us to act quickly and decisively as needed," said Biden.

Madiha Afzal, a fellow at the Brookings Institution, noted that Biden's address was meant to counter the growing perception that the Taliban are gaining ground and the collapse of the Afghan government is imminent, especially given his emphasis that the U.S. intelligence community has not reached the conclusion that Afghanistan's government will collapse.

"Those comments from Biden were a clear pushback on the growing perception, given the losses of the last few days, that the Afghan security forces will not be able to put up any significant fight against the Taliban, and that the Taliban will take over militarily sooner rather than later," she said.

"He also sought to reassure his audience that the U.S. had done all it could do -- including training the Afghan security forces -- and it could do nothing else militarily that would ensure a different outcome."

Michael Kugelman, the director of the Asia program at the Wilson Center in Washington, said that Biden's address displayed a clear shift of priorities for the U.S.

"Biden sought to provide reassurance, mainly to an Afghan audience, about continued U.S. support to Afghanistan," said Kugelman.

"But let's be clear: Biden believes the U.S.'s biggest priorities lie elsewhere. For many Afghans -- and many Americans -- his pledges of continued assistance to Afghanistan, especially against the backdrop of an intensifying withdrawal, will ring hollow."

Biden's several references to the faltering negotiations between the Taliban and the Afghan government remained a central theme of his address.

"The mission hasn't failed ... yet," said Biden. "The question now is where do they go from here. The jury is still out. The likelihood that the Taliban will be overrunning everything and owning the whole country is highly unlikely."