After 20 years, the U.S. military experiment in Afghanistan has ended.

It was an end marred by chaos and death as the Biden administration rushed to evacuate thousands of people fleeing Taliban rule and terrorists seized on the disorder to kill nearly 200 Afghans and 13 U.S. troops.

But despite repeated pleas from lawmakers in both parties to extend the mission until everyone who wanted to could get out, President Biden stuck with his deadline to withdraw.

That deadline came just before midnight Monday in Kabul, or 3:29 p.m. in Washington, D.C., as the last American troops departed, leaving Afghanistan without a U.S. military footprint for the first time in two decades and putting control back in the hands of the very group the invasion had ousted.

“There’s going to be many, many books written about why 20 years of American train-and-equip missions did not manage to solve this problem, did not manage to create a capable and strong Afghan national security force capable of fending off the Taliban,” said Emily Harding, a senior fellow at the Center for Strategic and International Studies.

“I don’t have the answer right now ... but there are certainly some lessons we need to learn as a country,” she added.

The withdrawal was the culmination of Biden’s announcement in April to get all U.S. troops out of Afghanistan by Sept. 11, the 20th anniversary of the terrorist attacks that triggered America’s longest war.

The deadline was later moved up to Aug. 31, and Biden and his team stuck with it despite chaos on the ground and uncertainty over whether evacuations would move quickly enough to get all Americans out of the country.

About 2,500 U.S. troops were in Afghanistan when Biden announced the withdrawal, most of whom left by July. But the administration this month rushed in about 6,000 U.S. troops to facilitate evacuations.

While Biden officials initially touted a safe and orderly withdrawal, pandemonium broke out after the Taliban, in roughly the span of a week, went from winning their first provincial capital to seizing control of Kabul, unwinding the past two decades of U.S. work to establish a Western-style democracy and leaving those who helped the U.S. military, women, minorities and other vulnerable Afghans scrambling to escape.

Over the course of evacuations, U.S. military and coalition aircraft flew out more than 123,000 civilians, including 6,000 Americans.

“I was very conflicted, actually,” U.S. Central Command chief Gen. Frank McKenzie, the official tasked with announcing the end of the withdrawal to the American public, said of the war’s end on Monday. “I'll have days ahead to actually think about that. There was just so much going on in this headquarters, and we were so completely focused on getting our troops out and, in the days before, getting our citizens out and all Afghans to the best of our ability that I did not have a lot of time for reflection. I'm sure I will do that in the future.”

Biden, who during his time as vice president advocated for a smaller military presence in Afghanistan, has been steadfast in his belief that it’s time for the U.S. to end its on-the-ground efforts there. He has been unwavering in that viewpoint even as bedlam outside the Kabul airport led to heart-wrenching images and after a bombing last week killed 13 U.S. service members.

“I have never been of the view that we should be sacrificing American lives to try to establish a democratic government in Afghanistan, a country that has never once in its entire history been a united country, and is made up — and I don't mean this in a derogatory — made up of different tribes who have never, ever, ever gotten along with one another,” Biden told reporters last week after the bombing.

“As I've said 100 times: Terrorism has metastasized around the world; we have greater threats coming out of other countries a heck of a lot closer to the United States,” Biden added. “We don't have military encampments there; we don't keep people there. We have over-the-horizon capability to keep them from going after us.”

Biden has not publicly addressed the drawdown in Afghanistan at length in the days since the 13 service members died, though he released a written statement Monday evening. He attended a dignified transfer of their remains at Dover Air Force Base on Saturday, and he is expected to speak to the nation on Tuesday about the end of the war.

White House press secretary Jen Psaki said Monday that Biden stands by his decision to end the nation’s longest war.

“Because if he had not, his view and the view of many experts and military out there is we would have sent tens of thousands, potentially, or thousands at least more troops back into harm’s way, risking more lives and more people to fight a war the Afghans were not willing to fight themselves,” Psaki said. “Nothing has changed in that regard.”

The final days of the evacuation were focused on keeping U.S. troops safe after the airport attack as well as “demilitarizing,” or disabling, equipment left behind at the airport, including an air defense system known as C-RAM, 70 mine-resistant ambush protected vehicles, 27 Humvees and 70 aircraft.

Underscoring the danger of the final hours of the operation, five rockets were fired toward the Kabul airport on Monday. No casualties were reported as U.S. air defenses intercepted one rocket and three others landed outside the airport, but one rocket made it inside the perimeter.

Even as the administration touted the end of what had become the largest noncombatant evacuation operation in U.S. military history, officials acknowledged some Americans and Afghan partners were left behind, despite earlier promises from Biden to evacuate every American who wanted to leave.

“We did not get everybody out that we wanted to get out, 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,” McKenzie said. “It's a tough situation.”

Secretary of State Antony Blinken said “under 200 and likely closer to 100” U.S. citizens remain in Afghanistan but that officials are still working to determine the exact number.

It’s also unclear, amid the rush of Afghans who sought to get out, how many Special Immigrant Visa (SIV) applicants and their families were evacuated and how many were left behind.

SIVs are reserved for Afghans who helped the U.S. government during the war. There were an estimated 80,000 applicants and family members in Afghanistan before evacuations started.

Pentagon press secretary John Kirby said Monday morning he did not have a breakdown of how many evacuees were SIVs but that he expected “there will be a time when this is complete that the State Department can do the math and figure this out.”

Blinken, who did not take questions after his Monday night speech, did not offer a number for SIV evacuees.

“We've gotten many out, but many are still there. We will keep working to help them. Our commitment to them has no deadline,” Blinken said. “We will hold the Taliban to its pledge to let people freely depart Afghanistan.”

But Biden has taken bipartisan criticism for not starting evacuations earlier and not staying until everyone who wants to leave has.

"This national disgrace is the direct result of President Biden’s cowardice and incompetence,” Sen. Ben Sasse (R-Neb.) said in a statement Monday. “The president made the decision to break our word to our Afghan partners. The president made the decision to tell one lie after another as the crisis unfolded. The president made the morally indefensible decision to leave Americans behind. Dishonor was the president’s choice. May history never forget this cowardice.”

Sen. Jeanne Shaheen (D-N.H.) reiterated Monday her “chief concern with what I believe was a premature U.S. withdrawal” that “the progress we built over twenty years would unravel without verifiable assurances of a stable, secure government in Afghanistan.”

“It is paramount that the United States remain engaged with our international partners to evacuate remaining U.S. citizens and Afghan allies, prevent Afghanistan from becoming a safe haven for terrorists, push the Taliban for accountability and ensure that the ISIS-K terrorists culpable for the murders of American service members pay for their crimes,” she said in a statement.

But administration officials have fiercely defended their evacuation efforts, and Blinken on Monday stressed that lessons must be learned from the 20-year conflict.

“This moment also demands reflection,” he said. “The war in Afghanistan was a 20-year endeavor. We must learn its lessons and allow those lessons to shape how we think about fundamental questions of national security and foreign policy. We owe that to future diplomats, policymakers, military leaders, service members. We owe that to the American people.”

The Biden administration has also continued to face questions about how now it will keep terrorist threats in check, particularly after last week’s Kabul airport attack conducted by the Afghan branch of ISIS.

In the wake of that attack, the U.S. military has conducted two strikes with drones based outside Afghanistan — one Saturday it said killed two “high-profile” ISIS-K targets and another Sunday it said thwarted a suspected car bomb in Kabul. Officials have pointed to those strikes as proof the U.S. military is capable of “over-the-horizon” counterterrorism operations.

“The president has made it clear that our combat mission, the war we have been fighting in Afghanistan, that that's gonna end,” Kirby said. “But what's not going to end is our commitment, especially here at the Defense Department, to protect the American people from threats and particularly from any terrorist threat that could emanate from Afghanistan again.”

But Sunday’s strike also underscored a risk of such drone strikes as reports from the ground in Kabul indicated at least 10 civilians from one family, including two 2-year-old girls, were killed. Kirby said the Pentagon is investigating the reports but is “not in a position to dispute [them] right now.”

“For two decades the United States has carried out strikes with no accountability to the public for how many civilians were killed by U.S. actions in Afghanistan and other countries,” Paul O’Brien, the executive director of Amnesty International USA, said in a statement Monday. "This airstrike is a glimpse into the future U.S. involvement in Afghanistan if the Biden administration pushes ahead with an ‘over the horizon’ counter-terrorism program that does not prioritize civilian protection.”