Over the last two decades, more than 750,000 American service members have been deployed to Afghanistan. Amid the chaos in the country as the U.S. pulls out, many are wondering if their sacrifices were wasted. 

"I'm on the verge of breaking down crying because this is just like we gave so much and just like that is wiped out," said Juan Dominguez, who was a Marine lance corporal when he was sent to the Taliban heartland of Helmand Province in 2010. 

It was there that his battalion lost 25 men. Dominguez, who lost both legs and an arm, said his thoughts are with the Afghans who helped the Marines and are now at the mercy of the Taliban. 

"We have seemingly turned our backs on them and just pray, pray for them, because what else can we do at this point? Because our government didn't help them get that safe passage out," he said. 

Read more here or watch the full story in the video player below: 

As the U.S. ends its longest war in Afghanistan, "CBS Evening News" anchor and managing editor Norah O'Donnell reflects on the enormous costs of the conflict.

America's 20-year war in Afghanistan is ending as it started: with the Taliban in charge. The fall of Kabul has led to chaos at the airport as Afghans try to flee. Roxana Saberi reports from the Afghan capital. 

A U.S. Defense official said operations had resumed at Kabul's airport after flights were grounded for several hours after the chaos earlier in the day.

The U.S. has moved its embassy operations to Kabul's airport as they try to evacuate at-risk Afghans. 

President Biden said the Taliban takeover, happening so quickly after American troops left, showed that "no amount of military force would ever a deliver secure, united, and stable Afghanistan." Mr. Biden added the country is known as the "graveyard of empires."

Mr. Biden noted that he is the fourth president to preside over this war, and said "many missteps" were made over the past 20 years. 

"I will not pass this responsibility onto a fifth president," he said. "I will not mislead the American people that claiming just a little more time in Afghanistan will make all the difference. Nor will I shrink from my share of responsibility for where we are today and how we must move forward from here. I am president of the United States of American and the buck stops with me."

He said he is "deeply saddened" by the current situation, but he does not "regret" the decision to move forward with the withdrawal of troops.

President Biden described the scenes from Afghanistan as "gut-wrenching," and said the U.S. will continue to stand with the Afghan people. 

Mr. Biden reiterated that he has authorized 6,000 troops to assist with the departure of U.S. and allied personnel in Afghanistan and to evacuate at-risk Afghans. Mr. Biden said the troops are taking over air traffic control, have safely shut down the U.S. Embassy, and over the coming days will transport Americans and at-risk Afghans, many of whom aided Americans in the 20-year war.

He said, "We have made it clear to the Taliban that if they attack our personnel or disrupt our operation, the U.S. presence will be swift and the U.S. response will be swift and forceful. We will defend our people with devastating force if necessary." 

Mr. Biden described the current mission as "short in time, limited in scope and focused in its objectives." 

"Once we have completed this mission, we will have conclude our military withdrawal," he said. "We will end America's longest war after 20 long years of bloodshed."

President Biden said he stands "squarely behind my decision" to pull U.S forces out of Afghanistan, saying "there was never a good time to withdraw."

Mr. Biden conceded that the Afghan government's collapse had happened more quickly than anticipated as political leaders "gave up" and fled the country.

Mr. Biden said it is "wrong to order American troops to step up when Afghanistan's own armed forces would not."

"How many more generations of America's daughters and sons would you have me send to fight Afghanistan's civil war when Afghan troops will not?" Mr. Biden said. "How many more lives, American lives, is it worth, how many endless rows of headstones at Arlington National Cemetery?"

President Biden, in his White House speech, said the U.S. succeeded in its mission to kill Osama bin Laden, and "our mission in Afghanistan was never supposed to be nation-building."

"I've argued for many years that our mission in Afghanistan should be counter-terrorism, not nation-building," Mr. Biden said. He said he had opposed the surge in Afghanistan as vice president. 

Mr. Biden said the U.S. priority should always be counter-terrorism, and he listed a number of other international terror threats the country faces from groups including al Shabaab and ISIS.

"These threats warrant our attention and our resources," Mr. Biden said.

 About 100 U.S. embassy staff remain at Kabul airport, sources say 

DEK: Sources tell CBS News roughly 100 embassy staff are still at the airport. They are operating what remains of the embassy, as opposed to standing around waiting for a plane. They will be the last ones out.

But sources warned the 100 number is not exact, with two sources saying it is hard to get a headcount right now. 

— Alana Anyse, Fin Gomez and Christina Ruffini  

French President Emmanuel Macron addressed the French people on Monday, saying French nationals have been gradually evacuated in recent weeks. 

Two military planes and French special forces are on their way to Afghanistan to help with evacuations. The country's priority is the safety of French nationals and Afghans who worked with them, Macron said. Any French nationals left in Afghanistan should contact the embassy representatives to plan their repatriation.

About 600 Afghans have been evacuated to France, Macron said. 

Macron defended his country's involvement in Afghanistan, which took place from 2001-14, calling it "useful." "France only ever had one enemy there: terrorism," he said. Ninety French soldiers were killed in the conflict.

Macron said France will continue to fight terrorism, and "Afghanistan must not become the sanctuary for terrorism it used to be." He said the country will be working closely with European allies, and U.K. Prime Minister Boris Johnson.

Pentagon spokesman John Kirby said that 22,000 at-risk Afghans will make their way to the U.S. 

Kirby said the Pentagon is looking at other installations in addition to Fort Lee, in Virginia, where the first round of SIV (Special Immigrant Visa) applicants and their families are being housed. The SIV applicants include thousands of Afghans who worked with the U.S. military and their families. In a third country, a battalion-sized unit is also prepared to process and provide medical care for up to 8,000 at-risk Afghans. 

President Joe Biden will make his first public address about the ongoing situation in Afghanistan on Monday afternoon. In the video above, CBS News correspondent Christina Ruffini discusses what's happening in the country and the administration's latest efforts to ensure the safety of all Americans in Afghanistan.

President Biden, who will be speaking Monday afternoon, has been briefed numerous times by his national security team, according to a source familiar. 

Mr. Biden has received a battlefield update and a contingency planning update daily, the source said. He authorized the pre-positioning of the battalions in theater weeks ago, in the event they would need to be deployed quickly. Mr. Biden authorized the provision of close air support through August 31and ordered the sustainment of contractors at Kabul's airport to help the Afghan Air Force keep flying.

The source said there has been extensive contingency planning to be ready for this scenario. While everyone has been watching closely and not sugarcoating the situation, the national security team has been ready to present the president with maximum possible options and quickly execute on every decision the president has made, from redeploying forces temporarily to delivering a clear, unambiguous message to the Taliban about how the U.S. will protect Americans and Afghan allies as the mission is completed. 

The source said the White House led a series of planning meetings at the deputies level this summer on security at the embassy in Kabul, and convened a tabletop exercise in early August before the Taliban's rapid advances to pressure-test every element of planning through August 31, the deadline for withdrawal. 

— Weijia Jiang and Ed O'Keefe  

Pentagon spokesman John Kirby confirmed two Afghans had been killed at Hamid Karzai International Airport. Kirby said there were two separate security incidents where U.S. troops acted in self defense. He said he did not know if both deaths occurred in the same incident or separate incidents. 

Kirby also said preliminary reports indicated a U.S. soldier was wounded at the airport. There are 2,500 troops there now providing security as Americans evacuate.  

Evacuations are expected to resume Monday evening, Kirby said. The runway needs to be cleared for flights to resume.

President Joe Biden returned to the White House Monday to deliver remarks on the situation in Afghanistan — his first public comments on the matter in nearly a week as chaos and uncertainty unfolds in the region. CBS News senior White House and political correspondent Ed O'Keefe and CBS News correspondent Christina Ruffini joined CBSN to discuss the latest developments in the video above.

A Taliban spokesman promised on Monday that the group would not attack foreign diplomats, or foreign or Afghan humanitarian aid workers after storming back to power in Afghanistan.  

"We assure all diplomats, embassies, consulates, and charitable workers, whether they are international or national, that not only no problem will be created for them on the part of IEA [Islamic Emirate of Afghanistan], but a secure environment will be provided to them, Inshallah," said Suhail Shaheen in a post on his Twitter account.

Shaheen was the voice of the Taliban in Doha, Qatar, as it negotiated for years with the U.S. government to get American forces pulled out of Afghanistan.

Shaheen said the group's leaders had also ordered fighters not to attack or enter homes without permission, including those of former government officials. 

"Life, property and honor of none shall be harmed but must be protected by the Mujahedeen," he said.

While there have been reports of atrocities committed by Taliban militants during their swift takeover in the wake of the U.S. military withdrawal, it remained unclear on Monday how extreme they intended to be during this new rein of power, with the entire world watching closely.

President Biden is to address the nation live from the East Room of the White House on Monday afternoon to discuss the Taliban's dramatic toppling of the Afghan government in the wake of the U.S. military withdrawal.

CBS News will air a network Special Report to carry Mr. Biden's remarks at 3:45 Eastern, which will also be viewable on CBSN in the player at the top of this article.

General Frank McKenzie, the U.S. Central Commander and the American officer in overall charge of evacuations from Afghanistan, has met face-to-face with Taliban officials to warn them not to interfere with those efforts, his spokesman confirmed to CBS News' David Martin on Monday.

Evacuation flights from Kabul's international airport, which was secured on Sunday by a still-growing U.S. military contingent, were halted on Monday as hundreds of desperate Afghan civilians breached the perimeter and rushed onto the runway as planes tried to take off. 

A news outlet with close ties to the Taliban earlier cited an unnamed source as saying an "agreement" had been reached between the militant group and the U.S. military that would see the Taliban keep its fighters out of the airport until all American forces were withdrawn. That report did not mention the evacuation of U.S. and other civilians.

An official told Martin that the runway at Hamid Karzai International Airport was about to reopen to allow evacuation flights to resume. There were two U.S. C-17 transport aircraft circling the airport, waiting to land, about three hours after all flights were halted.

U.N. Secretary-General Antonio Guterres appealed to world leaders on Monday as the Taliban reasserted control in Kabul, saying: "We cannot and must not abandon the people of Afghanistan."

Guterres addressed the U.N. Security Council as it convened for an emergency meeting on Monday morning to discuss the humanitarian and refugee crisis unfolding in Afghanistan, and as diplomats from the 15-nation Council sought answers about what to do next.

With member states fearful of the Taliban's history of giving safe haven to al Qaeda and State affiliates, which have been detailed in U.N. reports, by Afghanistan commanders, and on Capitol Hill, the U.N. chief urged unity.

"The international community must unite to make sure that Afghanistan is never again used as a platform or safe haven for terrorist organizations," he said. "I appeal to the Security Council — and the international community as a whole — to stand together, work together and act together — and use all tools at its disposal to suppress the global terrorist threat in Afghanistan and guarantee that basic human rights will be respected."

With over 19 agencies in Afghanistan, including UNICEF, the children's agency; WFP, the food program; and UNOCHA, the humanitarian agency; and the U.N. Assistance Mission in Afghanistan, or UNAMA, the United Nations hopes to keep a presence in Afghanistan, and Guterres made that presence a clear objective.

"The United Nations is committed to supporting Afghans. We continue to have staff and offices in areas that have come under Taliban control."

"All of us have seen the images in real time. Chaos. Unrest. Uncertainty, and fear. Much lies in the balance. The progress. The hope. The dreams of a generation of young Afghan women and girls, boys and men," he said. "The following days will be pivotal. The world is watching."

Wyoming Representative Liz Cheney, a member of the House Armed Services Committee, told "CBS This Morning" on Monday that the U.S. government — both the Trump and Biden administrations — had created a "dangerous new phase in the war on terror" by effectively surrendering Afghanistan to the Taliban.

Cheney said the Trump administration's decision to unilaterally negotiate a withdrawal agreement with the Taliban, cutting the Afghan government out of the process, "actually was a surrender agreement" as it set out a clear date for the U.S. pull-out in exchange for promises from the Taliban, which, as Cheney noted, were never met.

"We never should have done that," Cheney said, "but President Biden never should have withdrawn forces."

"When you look at how we got to this point, certainly there is sufficient blame on both sides," said Cheney, a Republican and the daughter of former Vice President Dick Cheney. "We're watching unfold what it looks like when America adopts a policy of retreat, when America adopts a policy of surrender, and it makes us less safe and it's gonna make the war longer."

Asked whether she believed the U.S. should instead have maintained a permanent military presence in Afghanistan, Cheney said: "We need to determine, and I believe that it is the case, whether our security requires that we have sufficient forces to work with the Afghans... air support, counterintelligence, counterterrorism efforts, to prevent [terrorist] safe havens." 

As chaotic scenes played out at Kabul's international airport, forcing a temporary halt to evacuation flights, U.S. National Security Advisor Jake Sullivan told "CBS This Morning" that American troops were "working to secure that airfield" so planes could start flying again. 

"We intend to carry out those evacuation flights after securing the airport over the coming days," he said, adding that the administration still wanted to evacuate "thousands of people in total. That includes a significant number of Americans."

"We are still doing outreach to establish the total number of Americans in country," Sullivan said. 

President Joe Biden's National Security Advisor Jake Sullivan defended the administration's sudden withdrawal of staff from the U.S. Embassy in Kabul on Monday, telling CBS News that the Taliban's dramatic retaking of Afghanistan's major cities, including the capital, had taken the administration at least somewhat by surprise. 

Sullivan suggested that a decision by the Afghan security forces to stand down had enabled the Taliban to storm back into power so quickly.

"The speed was at the very fast end. It was not something that we expected to go that quickly," he said about the Taliban's seizure of Kabul, which sent the president fleeing into exile on Sunday. 

"But we had contingency plans in place for any eventuality, including a quick fall of Kabul," Sullivan insisted, pointing to the U.S. military presence at Kabul's airport as evidence of the planning.

"President Biden was not prepared to usher in a third decade of war and put U.S. troops in harm's way, fighting and dying to try to hold Afghanistan together when its own armed forces would not fight to hold it together," he said. "This is about hard choices, and the choice he [Biden] made, he believes was in the national security interest of the United States."   

"We had hoped that they [Afghan forces] would fight. We hoped that they would stand up. But it became clear as city after city fell that in fact the Afghan army was not prepared, despite billions of dollars and years of training and all of the advanced capabilities we provided, and that raises real questions about whether one more year or two more years or five more years would have made any difference in terms of U.S. troops staying in Afghanistan." 

The U.S. Embassy in Kabul — which had been completely evacuated by Monday as all staff were moved to the Afghan capital's airport for flights out of the country — warned any other Americans still in Afghanistan not to just show up at the airport hoping to escape on Monday. 

"The security situation in Kabul continues to change quickly, including at the airport," a Security Alert posted online and shared on the embassy's social media accounts said. It told U.S. nationals to shelter in place and fill out an online form requesting evacuation, but to travel to Kabul's Hamid Karzai International Airport only after receiving email confirming their space on a flight.

It wasn't clear whether flights were even taking off on Monday after a temporary halt amid chaos on the tarmac.

The Taliban sought to project soft power from the heart of Kabul on Monday after storming back to power in Afghanistan two decades after being ousted by the U.S.-led military invasion.

The group posted messages on social media urging Kabul residents to get in touch via a phone number or social media if they have any problems. Other Taliban officials posted video clips showing members greeting residents in Kabul's streets and taking selfies. 

Speaking to Al Jazeera TV, Taliban spokesman Mohammad Naeem said the group asked "all countries and entities to sit with us to settle any issues."

"We have reached what we were seeking, which is the freedom of our country and the independence of our people," he said. "We do not think that foreign forces will repeat their failed experience in Afghanistan once again."

U.S. military officials told CBS News' David Martin on Monday morning that all evacuation flights from Kabul's international airport had been temporarily halted amid chaos on the tarmac.

The pause in flights came amid reports that at least five people died at the airport as hundreds of Afghans tried to get onto departing aircraft carrying foreign nationals. It was not clear whether flights had been cleared to resume, or when they might.

Amid the mayhem there was also still no confirmation from the U.S. military about what actions had been taken by forces on the ground to try to control the crowds.

At least one person was seen falling off of a U.S. military plane right after it took off from Kabul's airport on Monday, as dozens of desperate Afghan civilians swarmed the tarmac hoping to escape their country on one of the aircraft evacuating foreign nationals.

Video posted to social media showed a person plummeting to the ground from a significant height after the plane left the runway at Hamid Karzai International Airport.

Hundreds of people swarmed onto the tarmac on Monday, and the U.S. C-130 transport plane had a large crowd running along with it as it took off, some clinging to wheels and anything else they could grab as it sped down the runway. 

Dozens of people tried to clamber onto a civilian aircraft earlier as it prepared to take off full of foreign nationals. 

There were unconfirmed reports that at least five people died in the chaos, and video posted online showed some bodies on the ground at the airport.

Afghanistan's civilian aviation authority sent airlines notices on Monday saying the "civilian side" of the country's primary international airport in Kabul had closed and that the military was in control of Afghanistan's airspace. 

Most major airlines, including British Airways and Virgin Atlantic, had already rerouted flights around Afghan airspace as the Taliban retook control of most of the country, including the Sunday seizure of the presidential palace in Kabul.

Early on Monday, United Airlines told CBS News that it was rerouting flights "due to the dynamic nature of the situation" in Afghanistan.

Russia and China both said Monday they're taking a wait-and-see attitude about establishing diplomatic relations with the Taliban.

CBS News' Svetlana Berdnikova reports that a special representative of Russian President Vladimir Putin, Zamir Kabulov, told the Ekho Moskvy radio station everything will depend on their actions.

"We will carefully look at how responsibly they will govern the country in the near future and, based on the results, the Russian leadership will draw the necessary conclusions," he said.

CBS News' Mary Illyushina  says Kabulov told the station  Russian Ambassador to Afghanistan Dmitry Zhirnov will meet with a representative of the Taliban Tuesday to discuss the security of the Russian diplomatic mission. The Taliban is banned in Russia.

China struck a more positive tone, saying it's willing to develop "friendly relations" with Afghanistan's new rulers, according to Agence France-Presse.

"China respects the right of the Afghan people to independently determine their own destiny and is willing to continue to develop ... friendly and cooperative relations with Afghanistan," AFP quotes foreign ministry spokeswoman Hua Chunying as telling reporters.

A Florida congressman who lost both legs while serving as a bomb disposal expert in Afghanistan in 2010 issued a scathing statement about the Biden administration's handling of the U.S. pullout.

Representative Brian Mast, a Republican, said, "I have been asking for months for answers on how the Biden Administration planned to execute this withdrawal, and now they've showed us: be so unprepared that all those who worked with us are going to be executed.

"Our allies, and their families, are on the run. They've been forced to burn the first thing they need to get a Special Immigrant Visa – proof that they worked with us – because that's also the proof the Taliban will use to behead them. And they can't just show up at the airport because the Taliban will be waiting there for them.

"This is truly the worst-case scenario. If they let me, I would go back tomorrow to help in any way possible."

As thousands of people evacuate Afghanistan amid a Taliban takeover, the fate of Afghan nationals who helped U.S. forces during the 20-year war is very much up in the air.

Many are applying for a special visa that would allow them to relocate to the U.S., but time isn't on their side.

Association of Wartime Allies co-founder Kim Staffieri spoke with CBSN's Michael George about their struggle to secure safe passage out of the country.

In 2001, five young women, all refugees from Afghanistan, talked about living under Taliban rule. 

The scramble to evacuate thousands of American citizens from Kabul as the Taliban closed in on the Afghan capital prompted criticism from some Republican lawmakers and other observers that the U.S. intelligence community had failed to anticipate how quickly the government would fall. 

Current and former intelligence officials pushed back on those accusations Sunday. 

"We have noted the troubling trend lines in Afghanistan for some time, with the Taliban at its strongest, militarily, since 2001," a senior U.S. intelligence official told CBS News. "Strategically, a rapid Taliban takeover was always a possibility."

"The question all along was whether the Afghan government and military would be cohesive enough and have the willpower needed to exercise its military capabilities to resist the Taliban. As the Taliban advanced, they ultimately met with little resistance," the official said. "We have always been clear-eyed that this was possible, and tactical conditions on the ground can often evolve quickly." 

The official said the intelligence community's focus would now be on preventing future terror attacks and supporting ongoing efforts to evacuate U.S. citizens and Afghan partners. 

Former CIA acting director and CBS News senior national security contributor Michael Morell said on Twitter that the developments in Afghanistan were not the result of an intelligence failure but of "numerous policy failures by multiple administrations." 

"Of all the players over the years, the Intelligence Community by far has seen the situation in Afghanistan most accurately," Morell wrote.