Top US general in Afghanistan turns over command in symbolic end to America's longest war

The top U.S. military general leading the withdrawal in Afghanistan stepped down during a ceremony in Kabul Monday, a sign that America's longest war is nearing its end.

Gen. Austin Scott Miller has commanded U.S. Forces−Afghanistan and the NATO-led Resolute Support mission since the summer of 2018. At Monday's ceremony, Miller handed his responsibilities off to Gen. Frank McKenzie, who leads U.S. Central Command (CENTCOM) from its headquarters in Tampa, Florida.

"I need to let you know that command of this coalition has been the highlight of my military career," Miller told a small audience at Resolute Support headquarters in Kabul.

"The countries that have served here, many have lost service members, civilians; our Afghan partners have lost service members, they've lost civilians," Miller said. "And as we've spoken about it previously, on this very ground with this group over time, our job is now just not to forget."

Defense Secretary Lloyd Austin thanked Miller for his leadership in Afghanistan in a Monday afternoon tweet.

"I want to thank Gen. Miller for three years of exceptional leadership in Afghanistan. That we have been able to secure our interests in Afghanistan, as well as those of our allies and Afghan partners, stands as testament to his operational acumen and strategic vision," he wrote on Twitter. "It’s also worth noting that we have conducted our retrograde safely and orderly, and the transfer in command from Gen. Miller to Gen. McKenzie does not signify the end of our drawdown process, only the next milestone. We remain on track to meet @POTUS' end of August goal."

A Pentagon spokesperson said Miller left Afghanistan following the ceremony and was traveling back to the United States.

As the U.S. military finishes its withdrawal from the country, an emboldened Taliban has ramped up attacks and gained ground.

"I'm one of the U.S. military officers who's had the opportunity to speak with the Taliban," Miller told the audience in Kabul. "I said, it's important that the military sides set the conditions for a peaceful and political settlement in Afghanistan, we can all see the violence that's taking place across the country, but we know that with that violence, that what is very difficult to achieve is a political settlement. So again, what I tell the Taliban is they're responsible too."

McKenzie, who traveled to Kabul to attend the handover ceremony, said that while Miller's departure is a milestone in the U.S. withdrawal, it also signifies "our renewed commitment to our Afghan partners."

"The most important thing that continues is our support to the people of Afghanistan and to its armed forces," McKenzie told the audience, which included Dr. Abdullah Abdullah, chief of Afghanistan's National Council for Reconciliation, and other Afghan officials.

"We are confident in you. We are confident you have what it takes to protect your country," McKenzie said. "Our support will be different than what it was in the past, but we know how much you love your country, and we know the sacrifices that you have made in the past and that you're going to be willing to make in the future to do that. You can count on our support in the dangerous and difficult days ahead. We will be with you."

McKenzie will maintain the ability to launch counter-terrorism operations from bases and ships outside of Afghanistan as needed, but a major post-war concern is maintaining the U.S. diplomatic mission

To that end, a detachment of about 650 U.S. troops will remain in the country indefinitely to protect the U.S. embassy as well as the airport in Kabul, which is critical to keep the mission running.

The embassy announced Sunday that it had resumed in-person interviews for immigrant visas, including those Special Immigrant Visas (SIVs) for Afghan translators, guides and other contractors who worked for the U.S. military and diplomatic missions.

Interviews are a key part of the visa application process, but they had been halted for a month because of a significant COVID-19 outbreak throughout Afghanistan, including the U.S. embassy there.

There are approximately 18,000 Afghans seeking SIVs: 9,000 who haven't finished their application and 9,000 who are waiting for the U.S. government to move their case forward, according to a State Department spokesperson.

This resumption does not yet extend to everyone. For now, the embassy is only rescheduling applicants who had their interviews postponed and appointment capacity remains limited, the embassy said Sunday.

In the meantime, the U.S. will relocate a group of SIV applicants and their families out of the country to safe locations to await their cases being processed, President Joe Biden confirmed last week. While he said those relocations will begin before the end of this month, it's still unclear how many applicants that will involve, where they will go and when.

Despite pressure from lawmakers and activists, the Biden administration has emphasized SIVs as the way to help Afghans whose lives are at risk, instead of evacuations. Secretary of State Antony Blinken has assigned 50 more staffers in Washington to help process paperwork and work through the enormous backlog.

But even with all that, critics said time is running out for the Afghans who risked theirs and their families' safety by working for the U.S., as American forces draw down and the Taliban gain control of more districts.

ABC News' Aleem Agha contributed to this report.