The withdrawal of the final U.S. troops from Afghanistan on Monday marks the end of the U.S. military’s 20-year mission in Afghanistan.

But for President Biden, the end of the “forever war” is more of an inflection point than an actual conclusion. The departure of forces kicks off a new phase of the United States’s entanglement in Afghanistan that could also prove perilous — and no less challenging for American leadership than the previous two decades.

Biden and his team now have to grapple with deep skepticism over whether the Taliban, which now rules Afghanistan, will keep its promises for a peaceful transition. It pledged not to seek revenge on the Afghans who worked with and aided Americans during the conflict, and to respect the rights of women — at least within the framework of the group’s interpretation of Islamic law. But many foreign policy experts and even Biden allies remain mistrustful of what, exactly, that means.

National security threats remain, such as whether a Taliban-controlled Afghanistan will again become a haven for terrorists eager to attack the United States.

And Biden and his team are facing a humanitarian crisis in the form of tens of thousands of Afghan refugees. As of Saturday, more than 117,000 people — the majority Afghan citizens — had been evacuated from Afghanistan, but they now face an uncertain future, including in the United States, where the response to resettlement has ranged from welcoming to wary to hostile.

The administration will also face questions about whether the United States did enough to ensure Americans and eligible Afghans were actually able to leave the country in the final days of the drawdown and transition to Taliban rule.

The Pentagon on Aug. 30 said that the United States had pulled out the last of its troops, signifying the completion of a 20-year war. (The Washington Post)

In a news conference Monday to announce the official completion of the U.S. withdrawal from Afghanistan, Gen. Kenneth F. McKenzie Jr., the U.S. Central Command chief, told reporters that the number of American citizens left behind in Afghanistan is in the “very low hundreds.”

But he also acknowledged what he described as “a lot of heartbreak associated with this departure.”

“We did not get everybody out that we wanted to get out,” McKenzie said. “But I think if we’d stayed another 10 days we wouldn’t have gotten everybody out that we wanted to get out, and there still would have been people who would have been disappointed with that. It’s a — it’s a tough situation.”

Biden issued a statement Monday thanking troops and officers who oversaw the final withdrawal “with no further loss of American lives.”

“The Taliban has made commitments on safe passage and the world will hold them to their commitments,” Biden said in the statement, noting a U.N. resolution passed earlier in the afternoon to urge the Taliban to follow through on promises to allow Afghans to depart the country.

Biden plans to deliver remarks Tuesday regarding the end of the conflict.

The challenges his administration now face are both logistical and political. The chaotic withdrawal — including the deaths of 13 U.S. service members in a suicide bombing in Kabul last Thursday — has already threatened to undermine Biden’s core message of restoring calm and competency to governing, and Democrats are increasingly fearful of backlash further eroding their midterm election prospects. Some House Democrats have even begun privately discussing whether Secretary of State Antony Blinken and national security adviser Jake Sullivan should be fired over the recent mayhem in Afghanistan.

Ben Rhodes, a deputy national security adviser to former president Barack Obama, said the Biden administration is facing two pressing issues — the potential humanitarian crisis for the people of Afghanistan, and the counterterrorism threat in a region that has long been a safe haven for terrorists and where the United States now will have far less on-the-ground intelligence-gathering capabilities.

“In Afghanistan, they simultaneously have to make the case that it was right to end the war while demonstrating that they can also manage the aftermath in terms of counterterrorism and humanitarian challenges,” Rhodes said. “It’s not just enough to have ended the war. You need to show you are managing the aftermath.”

Danielle Pletka, a senior fellow at the American Enterprise Institute, a conservative think tank, said the administration’s withdrawal from Afghanistan leaves “endless unanswered questions.”

“What about those left behind? What about the presence of terrorist groups in Afghanistan? What are we going to do about the resistance to the Taliban? Support?” Pletka wrote, in response to emailed questions. “It will be up to the Taliban to dictate what happens next; they’re in the driver seat. The one thing we know is their track record: terrorists will find a welcome home where they recorded a victory against the United States.”

The Biden administration has regularly stressed how many people have been evacuated from Afghanistan so far, and on Monday, White House press secretary Jen Psaki again highlighted the evacuation numbers, noting that more than 120,000 people already have been evacuated, including 6,000 Americans and their families since mid-August.

Psaki also said that despite some setbacks — such as probably leaving some U.S. military equipment behind, where it will fall into Taliban hands — the administration is optimistic that it can still help influence the Taliban’s behavior.

“We have an enormous amount of leverage, including access to the global marketplace, which is not a small piece of leverage to the Taliban, who are now overseeing large swaths of Afghanistan,” Psaki said.

Still, the tumultuous weeks leading up to Tuesday’s final withdrawal deadline underscore the looming headaches for the Biden administration. Kabul, the capital of Afghanistan, fell to the Taliban far more quickly than U.S. intelligence anticipated, providing initial images of chaos and fear from city’s airport that were broadcast across the world.

On Thursday, a suicide bomber killed at least 170 people, including 13 U.S. service members, in an attack at the Kabul airport’s Abbey Gate.

And a U.S. military strike Sunday on a vehicle a mile from the Kabul airport that posed an “imminent” Islamic State threat killed civilians, as well, including children, according to officials in Afghanistan.

Ian Bremmer, president of Eurasia Group, a global risk consultancy, said next Saturday’s 20th anniversary of the Sept. 11 attacks also could provide the Taliban with a propaganda opportunity — and the Biden administration with the sort of problematic images they are hoping to avoid. The Taliban are likely, Bremmer suggested, to “be parading equipment that is American through the capital city and flying a Taliban flag on the embassy.”

“The Taliban have just won everything they could possibly win,” Bremmer said. “That’s not good for the U.S.”

There’s also the question of whether the Taliban can become an effective government, said retired Lt. Gen. Douglas Lute, former U.S. ambassador to NATO.

“We don’t  really  have a Taliban government — we have the remains of the Taliban insurgency,” he said. “It’s one thing to overthrow a government and another thing to govern.”

Bremmer said withdrawing the final U.S. troops from Afghanistan may ultimately give Biden the ability to move on from the troubled region, especially from an American public that — at least in theory — largely agreed with his decision to bring American service members home.

“Not to be too crude about it, but it’s kind of the end of when Americans pay serious attention because we no longer have troops on the ground and we will have basically gotten out every American that wants to get out,” Bremmer said.

Still, Republicans are eager to tie Biden to what they describe as a disastrous exit from the country. Rhodes said he expects Biden’s political opponents to take the confluence of crises facing the administration — from the still-raging pandemic to the “scary images in Afghanistan” — and “just kind of paint a narrative over the course of the next year of the scariest version of reality that they try to blame on Biden, even though most of it is out of his control.”

Pletka offered an even more stark assessment: “If people are right — and I think they are — this is an inflection point for the Biden presidency, the prism through which we judge all of his actions,” she wrote. “Ill-considered, ideologically rigid, mindless of the implications for the United States. If Biden is lucky, he will be Jimmy Carter. Unlucky, and he will have invited another 9/11.”

Anne Gearan contributed to this report.