KABUL, Afghanistan — Three major cities in western and southern Afghanistan were confirmed on Friday to have fallen to the Taliban, as the insurgents’ race to take control of the country accelerated.

The Taliban seized Lashkar Gah, the capital of Helmand Province, on Friday morning after a weekslong battle that left parts of the city in ruins, hospitals filled with the wounded and dying, and residents asking what would come next under their new rulers. Hours earlier, the insurgents had captured Herat, a cultural hub in the west, and Kandahar, the country’s second-largest city, where the Taliban first proclaimed their so-called emirate in the 1990s.

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Taliban Forces Sweep Across Afghanistan

Over the past week the Taliban have captured more than a dozen of Afghanistan’s provincial capitals, positioning them well to attack the capital, Kabul, just weeks before U.S. troops are expected to end their military mission there.

We’re focused on he security situation that we face now, which again, we’ve acknowledged is deteriorating. We are certainly mindful of the advances that the Taliban have made in terms of taking over yet — an increased number of provincial capitals. And the other thing I’d say is that no potential outcome has to be inevitable, including the fall of Kabul.

The speed of the cities’ collapse, combined with American officials’ announcement Thursday that they would evacuate most of the United States Embassy, has deepened the sense of panic across the country as thousands try to flee from the Taliban advance.

Only three major Afghan cities — the capital, Kabul, Jalalabad and Mazar-i-Sharif — remain under government control, and one is under siege by the Taliban. With the collapse of both Lashkar Gah and Kandahar, the Taliban now effectively control southern Afghanistan, a powerful symbol of their resurgence, just weeks before the United States is set to completely withdraw from the country.

Over the past week, the Taliban have taken one Afghan city after another in a rapid offensive that has left them well positioned to attack Kabul. The government’s forces appear close to a complete collapse. Some American officials fear that the Afghan government will not last another month.

On Friday, the Taliban also seized Pul-e-Alam, the provincial capital of Logar Province, south of Kabul, and Firoz Koh, the provincial capital of Ghor Province, in central Afghanistan.

“Sporadic clashes happened last night, but no serious resistance was reported,” said Gul Zaman Naeb, a member of Parliament representing Ghor Province. “When the people woke up this morning, they saw Taliban fighters in the streets and government offices.”

Helmand Province is a volatile swath of territory, much of which the Taliban have controlled since 2015. In recent months, the Afghan government has struggled to hold ground there, and recent airstrikes in the region by the United States and Afghan air forces failed to stop the Taliban offensive.

Lashkar Gah, Helmand’s capital, has been on the brink of disaster for more than a decade. Helmand has long been home to the Taliban, who spread to the province after the group’s rise in neighboring Kandahar in 1994 and proceeded to make millions there off the illicit sale of opium poppies.

The fall of Lashkar Gah is a sad coda for the American and British military missions in Helmand that, combined, lasted over a decade. Both countries focused much of their efforts on securing the province, losing hundreds of troops to roadside bombs and brutal gunfights there.

Kandahar, in particular, is a huge prize for the Taliban. It is the economic hub of southern Afghanistan, and it was the birthplace of the insurgency in the 1990s, serving as the militants’ capital for part of their five-year rule. By seizing the city, the Taliban can effectively proclaim a return to power, if not complete control.

On Friday, officials from Uruzgan and Zabul, two provinces long considered part of the Taliban’s heartland, said that local elders in both were negotiating a complete handover of the territory to the insurgent group.

Taimoor Shah contributed reporting from Kandahar, and Sharif Hassan from Kabul.

Several of his close political associates have surrendered to the Taliban without a fight or fled into exile. His army has all but collapsed, and the warlords he was counting on have proved ineffectual or are bargaining for their lives.

Afghanistan’s president, Ashraf Ghani, is more isolated than ever, facing pressure to step aside — and not just from the Taliban. His dominion is shrinking by the day. He governs the capital, Kabul, two other cities in the north and east, and pockets in the interior.

Yet Mr. Ghani is stubbornly clinging to office.

In a brief recorded speech televised early Saturday afternoon, Mr. Ghani promised to “prevent further instability” but did not resign. With Taliban forces having captured Pul-i-Alam, another provincial capital — this one only 40 miles from Kabul — Mr. Ghani said he had begun “extensive consultations at home and abroad” and that the “results” would soon be shared. He said “remobilizing” Afghanistan’s defense forces was a priority.

On Wednesday, he flew to one of his loyalist redoubts, the northern city of Mazar-i-Sharif, in attempts to rally pro-government forces; the city fell to the Taliban on Saturday night. On Thursday, officials said, he spoke by phone with Secretary of State Antony J. Blinken and Defense Secretary Lloyd J. Austin III. On Friday, he was said to be leading a national security meeting in the Kabul presidential palace.

The Afghan president’s options appear limited. He has little discernible support at home or from his former foreign backers. Street demonstrations supporting his army quickly fizzled out.

Thousands of his soldiers are surrendering, deciding that Mr. Ghani is not “worth fighting for,” Omar Zakhilwal, a former finance minister, tweeted on Friday.

Far from hinting at resignation, the president has only suggested that he would not run for re-election if the Taliban agreed to elections. Their battlefield rampage appears to have made the offer irrelevant. As his country slips away and provincial capitals fall, Mr. Ghani and his advisers have said little, sometimes even refusing to acknowledge the losses.

Even Mr. Ghani’s substantial corps of bodyguards, said to number in the thousands, poses a potential threat. Many are from villages now controlled by the Taliban.

Leading Afghanistan is a dangerous business. For more than a century, most Afghan rulers have been killed or have died in exile, the Boston University anthropologist Thomas Barfield pointed out.

Still, if — as seems increasingly likely — Mr. Ghani is deposed by the Taliban, he can lay claim to a singular distinction. “This will be the first insurgency that has ever driven a Kabul government from power that has also had the backing of a foreign power,” Mr. Barfield said.

The last time the Taliban seized control, in 1996, one former ruler wound up swinging from a noose on a lamppost in downtown Kabul, and the other fled hundreds of miles to the north to govern a postage-stamp rump state for five years.

Mr. Ghani shows no signs that the cruel lessons of the past sway him any more than the uncertain present and the fearful future.

“He’s hunkering down,” said Torek Farhadi, a former Afghan presidential adviser. “He’s refusing to admit the reality. The news is relayed to him through a filter.”

Mr. Farhadi added that “trusted lieutenants surrendered just this morning,” referring to the recent capitulations of governors Mr. Ghani had appointed in Ghazni and Logar provinces.

“He’s at risk from his own bodyguards,” Mr. Farhadi said. “This is how it happens in Afghanistan. The last days of any leader are like this.”

Mr. Ghazni’s youthful finance minister, Khalid Payenda, fled the country several days ago.

Leadership characteristics that in the past merely annoyed his fellow citizens — Mr. Ghani’s refusal to delegate authority or listen to others more knowledgeable than himself, especially on military matters, according to those who know him — are now proving lethal to the Afghan state.

“He is isolated, confused and deeply mistrustful of everyone,” said Tamim Asey, a former deputy minister of defense. “He doesn’t know how to reverse this.”

Unless a compromise can be reached, Mr. Asey said, “I would say that Kabul could become a blood bath very soon.”

The Taliban have said that the fighting will not end unless Mr. Ghani is removed. As the “polarizing figure,” in Mr. Farhadi’s words, Mr. Ghani has “demeaned the Taliban time and time again, saying, ‘You are the stooges of the Pakistanis.’” In return, the Taliban see him as the “stooge” of the Americans.

Analysts place much of the blame for the current disaster on the head of Mr. Ghani, a former World Bank anthropologist and published author with an outsize faith in his own intellect.

The Americans tried to construct republican institutions on Afghan soil, but they proved to be a flimsy facade. Instead, they helped bring Mr. Ghani to power through political deals struck during a contested presidential vote. Since then, Mr. Ghani has personalized power to disastrous effect.

He needed the militias in the north and west, yet showed contempt for their leaders. On Friday, Ismail Khan, a key militia leader in the western city of Herat, surrendered to the Taliban.

Mr. Ghani “did not take advice from anybody,” said Mr. Barfield, of Boston University. “If he had delegated power to the military, it might have been saved. Now, it’s a case of reality biting.”

While Afghanistan’s future seems more and more uncertain, one thing is becoming exceedingly clear: The United States’ 20-year endeavor to rebuild Afghanistan’s military into a robust and independent fighting force has failed, and that failure is playing out in real time as the country slips into Taliban control.

The Afghan military’s disintegration first became apparent months ago, in an accumulation of losses that started even before President Biden’s announcement that the United States would withdraw by Sept. 11.

It began with outposts in rural areas where hungry and ammunition-depleted soldiers and police units were surrounded by Taliban fighters and promised safe passage if they surrendered and left behind their equipment. That gave the insurgents more and more control of roads, and then entire districts.

As positions collapsed, the complaint was almost always the same: There was no air support, or they had run out of supplies and food.

Even before that, the systemic weaknesses of the Afghan security forces were apparent.

And when the Taliban started building momentum after the United States’ announcement of its withdrawal, it only increased the belief that the security forces — fighting for President Ashraf Ghani’s government — weren’t worth dying for. In interview after interview, soldiers and police officers described moments of despair and feelings of abandonment.

BRUSSELS — Afghanistan’s rapid unraveling is raising grumblings about American credibility, compounding the wounds of the Trump years and reinforcing the idea that America’s backing for its allies is not unlimited.

The Taliban’s lightning advance comes at a moment when many in Europe and Asia had hoped that President Biden would reestablish America’s firm presence in international affairs, especially as China and Russia angle to extend their influence.

“When Biden says ‘America is back,’ many people will say, ‘Yes, America is back home,’” said François Heisbourg, a French defense analyst.

“Few will gang up on the U.S. for finally stopping a failed enterprise,” he said of the war in Afghanistan. “Most people would say it should have happened a long time ago.’’

But in the longer run, he added, “the notion that you cannot count on the Americans will strike deeper roots because of Afghanistan.’’ The United States has been pulling back from military engagements abroad since President Obama, he noted.

Doubt about American commitment will be felt especially strongly among countries where China and Russia are vying for power.

“The sudden withdrawal from Afghanistan after 20 years and so much investment in lives and effort will see allies and potential allies around the world wondering whether they have to decide between democracies and autocracies, and realize some democracies don’t have staying power anymore,” said Tom Tugendhat, chairman of the British Parliament’s Foreign Affairs Committee.

European nations have never viewed Afghanistan as a vital national interest, but it has become more of a concern as more people flee violence and poverty there, and seek refuge in Europe.

European leaders, like others around the world, are worried about how long the Afghan government will last, about what will happen to women and girls and judges and the media under a renewed Taliban rule, and about a new wave of migration.

For NATO, the mantra was always “in together, out together.” Once President Biden decided to pull the plug, NATO troops also began leaving at speed; there is little appetite for returning.

In Britain, which fought hard in Afghanistan and has a long history of involvement in the country, there is more chagrin and even anger.

Lord David Richards, who was chief of defense from 2010 to 2013, criticized his government for moving so quickly to evacuate Britons. He told “BBC Newsnight” that the evacuation “is a tacit, explicit really, admission of a dismal failure of geostrategy and of statecraft.”

As the Taliban capture provincial capitals with alarming speed, the Pentagon is moving 3,000 Marines and soldiers to Afghanistan and another 4,000 troops to the region to evacuate most of the American Embassy and U.S. citizens in Kabul.

It is a powerful sign of the deteriorating situation in the country as well as one that appears to reinforce President Biden’s order to shut down America’s longest war.

The Biden administration is bracing for a possible collapse of the Afghan government within the next month, administration and military officials said.

The Taliban’s rapid advance across the north, and Afghan security forces’ battle to defend ever shrinking territory in the south and west, has forced the Biden administration to accelerate plans to get Americans out.

Mr. Biden, after meeting with his top national security advisers on Wednesday night and again Thursday morning, also ordered additional expedited flights out of the country for Afghans who have worked with the United States, so that their applications for special immigrant visas could be evaluated.

The embassy sent the latest in a series of alarming alerts, urging Americans to “leave Afghanistan immediately using available commercial flight options.”

And in Washington, the State Department spokesman, Ned Price, announced what he described as a drawdown of an unspecified number of civilians among the roughly 4,000 embassy personnel — including about 1,400 American citizens — to begin immediately.

“As we’ve said all along, the increased tempo of the Taliban military engagements and the resulting increase in violence and instability across Afghanistan is of grave concern,” he said. “We’ve been evaluating the security situation every day to determine how best to keep those serving at our embassy safe.”

But, Mr. Price added, “Let me be very clear about this: The embassy remains open.”

American negotiators are also trying to extract assurances from the Taliban that they will not attack the U.S. Embassy in Kabul if they take over the country’s government, three American officials said.

The estimate that Kabul could fall in 30 days is one scenario, and administration and military officials insist that it might still be prevented if the Afghan security forces can muster the resolve to put up more resistance. But while Afghan commandos have managed to continue fighting in some areas, they have largely folded in a number of northern provincial capitals.

After a bracing string of battlefield victories, Taliban forces are seeking to isolate Kabul, the Pentagon said Friday, taking over border crossings, highways and lines of revenue on its march through Afghanistan.

“You can see a certain effort to isolate Kabul,” said John Kirby, the Pentagon press secretary, at a news conference Friday. The move “is not unlike the way they’ve operated in other places of the country, isolating provincial capitals and sometimes being able to force surrender without necessarily much bloodshed.”

“We’re certainly concerned by the speed with which the Taliban has been moving,” he added. “And as we’ve said from the very beginning that this is it still is a moment for Afghan national security and defense forces, as well as their political leadership.”

The Taliban’s accelerated march toward Afghanistan’s capital underlies the urgency for American forces to assist with the evacuation of American and Afghan civilians, including State Department personnel and Afghans who have Special Immigrant Visas that permit them to leave the country for the United States. Three battalions of U.S. troops, or about 3,000 personnel, are being dispatched to Kabul for that effort.

“Nobody’s walking away from the fact that this is potentially dangerous,” Mr. Kirby said of the American mission, in which thousands of civilians are meant to move out of the country daily. “We’re all mindful of the perilous situation in Afghanistan.”

President Biden has vowed to end America’s longest war and withdraw its troops by the end of the month. And U.S. forces have sharply curtailed airstrikes in support of Afghan forces.

In spite of the rapid ascent of Taliban forces and the imminent danger to the capital Mr. Kirby said that the broader battle to secure Afghanistan would remain in the hands of the Afghan security forces.

“They have an air force, a capable Air Force,” he said. “They have organizational structure. They have the benefit of the training that we have provided them over 20 years. They have the material of physical that tangible advantages. It’s time now to use those advantages.”

KABUL, Afghanistan — A prominent Afghan warlord and former governor, who had resisted Taliban attacks in western Afghanistan for weeks and rallied many to his cause to push back the insurgent offensive, surrendered on Friday, officials said.

The surrender of the warlord, Mohammad Ismail Khan, is particularly important for the Taliban because he commanded a force that potentially posed a threat to the insurgents in the western region of the country — perhaps even a greater threat than Afghan government forces.

Mr. Khan’s surrender could kick off a trend among warlords and regional power brokers such as Mohammed Atta Noor, who is trying to defend the economic hub of Mazar-i-Sharif in the country’s north and has rallied militias for the city’s defense. During the civil war in the 1990s, it was common for warlord commanders to switch sides at the first sign of opportunity or survival.

Mr. Khan was a young army captain when the Soviet forces invaded Afghanistan in 1979. He rose to prominence after joining the mujahedeen, the U.S.-backed insurgents who fought against the Soviet-supported Communist government in his home of Herat.

He fought against the Taliban in the 1990s as part of the Northern Alliance in western Afghanistan. He was captured by the Taliban, and spent about two years as their captive.

Mr. Khan, now in his 70s, picked up his rifle last month, mobilized his militias, and fought alongside Afghan security forces in an attempt to push back the Taliban offensive on Herat, the capital of the province with the same name and the country’s third largest city.

He became a national celebrity for his steadfast resistance against the Taliban and thousands of Afghans commended him for his leadership against the insurgents, posting his photos on social media, calling him the “Lion of Herat.”

He initially succeeded, and pushed the Taliban fighters out of the city, but his battlefield victories were not enough to prevent its fall.

The Taliban finally seized control of Herat on Thursday night after a two-week siege, forcing Mr. Khan, top government officials and forces to retreat to the provincial airport and the army corps outside the city.

Mr. Khan and senior security officials including a deputy for the interior ministry, an army corps commander and an intelligence director, along with thousands of government forces, surrendered to the Taliban this morning.

The Taliban’s online media outlets later shared a video of Mr. Khan after the surrender.

“I hope all brothers can create a peaceful environment, so the war ends and we can have peace and stabilization in Afghanistan,” Mr. Khan told a member of the Taliban, from the back of a vehicle.

The fall of Herat, and the surrender of thousands of forces, come after the insurgents captured over half of the country’s 34 provincial capitals in just over a week. Among the fallen cities are the strategic southern Kandahar, the country’s second-largest city, and Kunduz, a commercial hub in the north.

It was a dramatic reversal for Mr. Khan, who was appointed as Herat governor after the fall of the Taliban in 2001, then served as a cabinet minister during Hamid Karzai’s presidency.

Along with other warlords, he was sidelined by President Ashraf Ghani, who once publicly dismissed Mr. Khan as not even being worth meeting.

The Taliban are claiming towns and territories across Afghanistan. With each victory, scrutiny is falling on the leaders of neighboring Pakistan.

For decades, Pakistan has served as a sanctuary for the Afghan Taliban, who have often crossed the countries’ rugged, 1,660-mile border with ease. Officials have acknowledged that Taliban fighters maintain homes and families in Pakistan, at a safe distance from the battlefields.

Now that the American military has declared its part in the Afghan war over, and the Taliban increasingly look as if they can capture the country, Washington is applying pressure on Pakistan to push for a negotiated settlement.

While voicing support for a peaceful solution globally, however, the government of Prime Minister Imran Khan has been quieter at home. It has not spoken out against pro-Taliban rallies within Pakistan. It also hasn’t condemned reported Taliban atrocities as the group marches toward Kabul.

The reason: A large number of Pakistanis, including military officers, describe a Taliban victory as inevitable. Some, including former military officials, are publicly cheering for one.

But a collapse in Afghanistan would carry risks for Pakistan, too, including a possible wave of refugees, and a boost to jihadist movements that target Pakistan’s government for attack.

“Pakistan is really in a bind,” said Elizabeth Threlkeld, a South Asia expert at the Stimson Center in Washington. “Even though Pakistan is really concerned about spillover violence and an influx of refugees, they want to keep the Taliban on side.”

In an interview with The New York Times in June, Mr. Khan said Pakistan had used the “maximum leverage it could on the Taliban.”

Pakistani officials deny helping the group militarily, insisting that during negotiations in Doha, Qatar, they pushed hard for peace talks with the Taliban. In public, they have echoed the line taken by the United States and other parties to the accord reached in Doha, warning that Afghanistan would become a pariah state if the Taliban took it by force.

But Pakistan has leverage that it is not bringing to bear, government officials in other countries say. It still allows Taliban leaders free movement into and out of the country and continues to serve as a safe haven where fighters and their families can receive medical care, they say.

Some critics, particularly in Afghanistan, accuse Pakistan of actively supporting the Taliban’s offensive, saying that the insurgents could not have mounted such a large effort without assistance. On social media, the hashtag campaign #SanctionPakistan has gained popularity in Afghanistan and among the diaspora.

Across Afghanistan, a mass exodus is unfolding as the Taliban press on in their brutal military campaign, bringing fears of a return to extremist rule or a civil war between ethnically aligned militias.

At least a quarter of a million Afghans have been forced to flee conflict since the end of May, 80 percent of them women and children, the U.N. refugee agency said on Friday.

“This is a staggering statistic,” Shabia Mantoo, a spokeswoman for the agency, told reporters in Geneva on Friday. “The most vulnerable are paying for what’s happening on the ground.”

More than 400,000 people have been driven from their homes since the start of the year, she added.

Many have flooded into tent camps in Kabul or crowded into relatives’ homes in cities, often the last islands of government control. Thousands more are trying to secure passports and visas to leave the country altogether. Others have crammed into smugglers’ pickup trucks in a desperate bid to slip illegally over the border.

Conflict is aggravating already dire food shortages brought on by a drought that is affecting the food supply for one-third of the population.

Some two million children are now in need of nutritional support, the U.N. food program said. It is already delivering food for four million Afghans but is now trying to scale up to deliver food aid to nine million people by December.

“We fear the worst is yet come and a larger tide of hunger is fast approaching,” the World Food Program spokesman, Tomson Phiri, said in Geneva.

Senior United Nations officials warned about the growing risks of a protracted civil war.

“We are particularly concerned about the shift of fighting to urban areas, where the potential for civilian harm is even greater,” Stéphane Dujarric, the spokesman for Secretary General António Guterres, told correspondents at the U.N. headquarters in New York on Thursday.

Mr. Guterres expressed hope that discussions between the Afghan government, the Taliban and international envoys would create a pathway to a negotiated settlement. But with the Taliban appearing intent on controlling the country, that goal appears increasingly elusive.

As the insurgents have pressed their offensive in recent weeks, the number of Afghans crossing the border illegally has shot up about 30 to 40 percent compared with the period before international troops began withdrawing in May, according to the International Organization for Migration. At least 30,000 people are now fleeing every week.

The sudden flight is an early sign of a looming refugee crisis, aid agencies warn, raising alarms in neighboring countries and in Europe.

Afghans already account for one of the world’s largest populations of refugees and asylum seekers — around three million people — and represent the second-highest number of asylum claims in Europe, after Syrians.

Now the country may be beginning another bloody chapter, but the new outpouring of Afghans comes as attitudes toward migrants have hardened around the world.

As the Taliban take control of city after city in Afghanistan, leading to fears that the capital, Kabul, could fall in a matter of weeks, the humanitarian crisis is growing as fighting escalates.

The Taliban’s capture of Kandahar, Afghanistan’s second-largest city, is a devastating blow to the Afghan government and has both symbolic and strategic resonance for the insurgent group.

The fight for Kandahar was, in some ways, the most important one for the country’s future so far.

The insurgents have been desperate to capture the city — the Taliban first took root in its neighboring districts in the 1990s before seizing the city itself and announcing their emirate. It is also the main city in Afghanistan’s ethnic Pashtun heartland in the south.

The government has been desperate to defend it, given its status as an emblem of the state’s reach and its role as an economic hub essential for trade to and from Pakistan through its checkpoints, bridges and highways.

The insurgents had been encroaching on Kandahar city, the capital of the province of the same name, for several weeks, capturing surrounding districts, before entering the city for the first time on July 9.

Taliban fighters at the time made their way into Kandahar’s Seventh Police District, seizing houses and engaging in gun battles with security forces. Commandos and other special forces units battled the insurgents, proceeding cautiously because the area is heavily populated, said Bahir Ahmadi, the spokesman for Kandahar’s governor.

The Afghan Air Force struck a number of Taliban positions in neighboring districts as the insurgents tried to push their way into the city.

U.S. military aircraft also struck Taliban positions in July in support of faltering Afghan government forces, in one of the first significant American reactions to the insurgents’ blistering advance across Afghanistan as U.S. troops withdrew.

At least one of the strikes was against Taliban positions in the city of Kandahar, slowing the Taliban’s advance. A month later, the fall of Kandahar will deeply shake a country still trying to come to terms with the Taliban’s rapid encroachment.

As the United States is moving thousands of troops to Afghanistan to evacuate U.S. citizens and embassy staff in Kabul, Britain said it would also deploy 600 military personnel to help its citizens leave the country.

The British authorities urged all British nationals to leave Afghanistan earlier this month, and on Thursday they said that the staff working at the embassy in Kabul had been reduced to a core team providing assistance to those still in the country.

“I have authorized the deployment of additional military personnel to support the diplomatic presence in Kabul, assist British nationals to leave the country and support the relocation of former Afghan staff who risked their lives serving alongside us,” Defense Secretary Ben Wallace said in a statement.

In a sign that the authorities fear the embassy may soon be unsafe, they said on Thursday that the core team remaining in Kabul would move from the embassy “to a more secure location.”

As the Taliban have seized one city after another, Britain has scrambled to evacuate former Afghan staff members who worked with British personnel. The defense ministry said that under a program launched in April, it had helped relocate more than 3,000 former Afghan staff members and their families, with 1,800 of them arriving over the last few weeks.

British combat troops withdrew from Afghanistan in 2014, after a 13-year intervention that at its peak saw 9,500 troops deployed across more than 130 bases in Helmand Province alone. On Friday morning, the Taliban seized Lashkar Gah, the province’s capital.

British opposition leaders said they supported the evacuation of British nationals, but at least one influential member of Prime Minister Boris Johnson’s Conservative Party said that sending troops to help with the withdrawal was “a sign of failure.”

Tom Tugendhat, a former member of the British armed forces who is now the chairman of the foreign affairs committee in Parliament, said on Twitter that “the decision to withdraw is like a rug pulled from under the feet of our partners.”

“Instead of a sustainable peace, incrementally building, we’re seeing a rout. Of course we are,” Mr. Tugendhat said.

With the Taliban sweeping across the country and the Afghan government controlling just three major cities, even experts have been surprised at how swiftly the nation is collapsing as American troops near the end of their withdrawal.

The Taliban’s summer-long military campaign has been followed by a lightning advance across a dozen provincial capitals, forcing widespread surrenders and retreats by Afghan government forces.

The Taliban military victories, especially in northern Afghanistan, where opposition to the militants has traditionally been strongest, have provided a violent coda to the U.S. military mission in America’s longest war.

As the United States prepares to leave the country, history is casting a long shadow.

Why did the United States invade Afghanistan?

Weeks after Al Qaeda attacked the United States on Sept. 11, 2001, President George W. Bush announced that American forces had launched attacks against the terrorist group and Taliban targets in Afghanistan.

Mr. Bush said the Taliban, which then governed most of Afghanistan, had rejected his demand to turn over Al Qaeda leaders who had planned the attacks from bases inside Afghanistan. He said he intended to bring Qaeda leaders to justice, adding, “And now the Taliban will pay a price.”

Even then, the president warned that Operation Enduring Freedom would entail “a lengthy campaign unlike any other we have ever seen.”

By December 2001, the Qaeda leader, Osama bin Laden, and other top commanders had fled to safety in Pakistan, a nominal U.S. ally. American forces did not pursue them, and Pakistan ultimately evolved into a safe haven for Taliban commanders and fighters, who in subsequent years crossed the border to attack American and Afghan forces.

Inside Afghanistan, American troops quickly toppled the Taliban government and crushed its fighting forces as 2001 drew to a close.

How did the mission in Afghanistan evolve?

After routing the Taliban, the United States and NATO pivoted to rebuilding a failed state and establishing a Western-style democracy, spending billions trying to reconstruct a desperately poor country already ravaged by two decades of war, first during the Soviet occupation of the 1980s and then during the proceeding civil war.

There were early successes. A pro-Western government was installed. New schools, hospitals and public facilities were built. Thousands of girls, barred from education under Taliban rule, attended school. Women, largely confined to their homes by the Taliban, went to college, joined the work force and served in Parliament and government. A vigorous, independent news media emerged.

But corruption was rampant, with hundreds of millions of dollars in reconstruction and investment money stolen or misappropriated. The government proved unable to meet the most basic needs of its citizens. Often, its writ barely extended beyond the capital, Kabul, and other major cities.

What happened on the battlefield?

The Taliban rebuilt their fighting capabilities, despite a steady influx of American and NATO troops, who sought to win over Afghans with promises of new schools, government centers, roads and bridges.

With the Taliban posing an enhanced military threat, President Barack Obama deployed thousands more troops to Afghanistan as part of a “surge,” reaching nearly 100,000 by mid-2010. But the Taliban only grew stronger, inflicting heavy casualties on Afghan security forces despite American combat power and airstrikes.

In May 2011, a U.S. Navy SEAL team killed Osama bin Laden in a compound in Abbottabad, Pakistan, where he had been living for years near a Pakistan military training academy. In June, Mr. Obama announced that he would start bringing American forces home and hand over responsibility for security to the Afghans by 2014.

By then, the Pentagon had concluded that the war could not be won militarily and that only a negotiated settlement could end the conflict — the third in three centuries involving a world power. Afghan fighters defeated the British army in the 19th century and the Russian military in the 20th century.

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U.N. Calls for Taliban to End Its Offensive in Afghanistan

The United Nations secretary general urged the Taliban to return to peace talks in Qatar, and highlighted the humanitarian crisis that had ensued since the insurgents began to quickly seize provincial capitals throughout the country.

Afghanistan is spinning out of control. The fighting between the Taliban and Afghan security forces in urban environments is causing tremendous harm. Hospitals are overflowing, food and medical supplies are dwindling. Roads, bridges, schools, clinics and other critical infrastructure are being destroyed. It is particularly horrifying and heartbreaking to see reports of the hard-won rights of Afghan girls and women being ripped away from them. Seizing power through military force is a losing proposition that can only lead to prolonged civil war or to the complete isolation of Afghanistan. I call on the Taliban to immediately halt the offensive and to negotiate in good faith in the interests of Afghanistan and its people. I hope that discussions in Doha, Qatar, between representatives of the Islamic Republic of Afghanistan and the Taliban supported by the region and the wider international community, will restore the best way to a negotiated settlement to the conflict. Only an Afghan-led, negotiated political settlement can ensure peace.

The U.N. secretary-general urged the Taliban to put down its weapons and return to peace talks in Qatar after the group’s offensive this week put them in control of a majority of the country.

“Seizing power through military force is a losing proposition that can only lead to prolonged civil war or the complete isolation of Afghanistan,’’ the secretary-general, António Guterres, said on Friday at a news conference where he took no questions. “I call on the Taliban to immediately halt the offensive and to negotiate in good faith in the interest of Afghanistan and its people.”

The Taliban seized three more major cities in the western and southern parts of the nation, adding to the territory under its control as part of a furious blitz to capture regional capitals after the United States military withdrew most of its forces last month.

The government of Afghanistan controls only three major cities now, including Kabul, the capital. And the United States said it will begin evacuating its citizens, a sign of the quickly deteriorating conditions there and the expectation that the Afghan government is on the verge of collapse.

“Humanitarian needs are growing by the hour,” Mr. Guterres said. “Hospitals are overflowing. Food and medical supplies are dwindling. Roads, bridges, schools, clinics and other critical infrastructure are being destroyed.”

He cited mounting civilian casualties and said he was seeing “severe restrictions on human rights” by the Taliban in areas they control.

“It is particularly horrifying and heartbreaking to see reports of the hard-won rights of Afghan girls and women being ripped away from them,” he added.