The War in Afghanistan: How It Started and How It Is Ending.

Just weeks before U.S. forces were set to complete their pullout from Afghanistan, the Taliban have seized the momentum, capturing the northern city of Kunduz on Sunday.

Kunduz, a city of 374,000, was the third provincial capital to be overtaken by Taliban in three days and its fall is a major blow to the Afghan government. The city, in a province of the same name, is a vital commercial hub near the border with Tajikistan.

The Taliban’s summer-long military campaign has forced widespread surrenders and retreats by Afghan government forces. By late July, the group had seized control of half of the country’s roughly 400 districts. Government troops abandoned scores of outposts and bases, often leaving behind weapons and equipment. In many cases, they surrendered without a fight, sometimes following the intercession of village elders dispatched by the Taliban.

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

In mid-April, President Biden, declaring that the United States had long ago accomplished its mission of denying terrorists a safe haven in Afghanistan, announced that all American troops would leave the country by Sept. 11.

A combat mission that has dogged four presidents — who reckoned with American casualties, a ruthless enemy and an often corrupt and confounding Afghan government partner — is at last coming to an end.

Mr. Biden conceded that after nearly 20 years of war, America’s longest on foreign soil, it was clear that the U.S. military could not transform Afghanistan into a modern, stable democracy.

Responding in July to critics of the withdrawal, the president asked: “Let me ask those who wanted us to stay: How many more? How many thousands more of America’s daughters and sons are you willing to risk?”

The United States is leaving behind about 650 troops to secure the U.S. Embassy in Kabul. Some are stationed at the international airport in the capital, alongside Turkish troops who have agreed to provide airport protection.

Why did the United States invade Afghanistan?

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

“These carefully targeted actions are designed to disrupt the use of Afghanistan as a terrorist base of operations, and to attack the military capability of the Taliban regime,” the president said.

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 Al 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.

In December 2001, the Taliban’s spokesman offered an unconditional surrender, which was rejected by the United States. Nearly 20 years later, the United States is pleading with the Taliban to agree to a cease-fire and negotiate a political settlement with the U.S.-backed Afghan government.

In May 2003, Defense Secretary Donald Rumsfeld announced an end to major combat operations in the country.

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.

In 2003, with 8,000 American troops in Afghanistan, the United States began shifting combat resources to the war in Iraq, launched in March of that year.

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.

With the war at a stalemate, Mr. Obama ended major combat operations on Dec. 31, 2014, and transitioned to training and assisting Afghan security forces.

Nearly three years later, President Donald J. Trump said that although his first instinct had been to withdraw all troops, he would nonetheless continue to prosecute the war. He stressed that any troop withdrawal would be based on combat conditions, not predetermined timelines.

But the Trump administration also had been talking to the Taliban since 2018, leading to formal negotiations that excluded the Afghan government, led by President Ashraf Ghani.

What is the status of peace talks?

In February 2020, the Trump administration signed an agreement with the Taliban that called for all American forces to leave Afghanistan by May 1, 2021. In return, the Taliban pledged to cut ties with terrorist groups such as Al Qaeda and the Islamic State affiliate in Afghanistan, reduce violence and negotiate with the American-backed Afghan government.

But the deal did not include enforcement measures to compel the Taliban to honor their promises. And with the Afghan government excluded from the U.S.-Taliban deal, relations with the United States were strained. The Trump administration pressured Mr. Ghani to release 5,500 Taliban prisoners while receiving little in return, further alienating the Afghan government.

After the deal was signed, the Taliban stopped attacking American troops and refrained from major terrorist bombings in Afghan cities. The United States reduced air support for government forces, generally restricting them to instances in which Afghan troops were in danger of being overrun.

The primary objectives of the 2020 deal were for Afghan leaders and the Taliban to negotiate a political road map for a new government and constitution, reduce violence and ultimately forge a lasting cease-fire.

But the government accused the Taliban of assassinating Afghan government officials and security force members, civil society advocates, journalists and human rights workers — including several women shot in broad daylight.

Because of their strong battlefield position and the U.S. troop withdrawal, the Taliban have maintained the upper hand in talks with the Afghan government, which began in September in Doha, Qatar, but have since stalled. The Pentagon has said the militants have not honored pledges to reduce violence or cut ties with terrorist groups.

The Biden administration says it continues to support peace talks, but the Taliban appear in no hurry to negotiate. Nor have they explicitly said they would agree to a power-sharing government, implying instead that they intend to fight for a monopoly on power.

Can Afghan security forces hold off the Taliban without U.S. troops?

Military and police units have been hollowed out by desertions, low recruitment rates, poor morale and leadership and the theft of pay and matériel by commanders. They have suffered high casualty rates, which American commanders have said are not sustainable.

Many of the country’s 34 provincial capitals, in both the north and the south, are under siege. Government counterattacks have retaken only a handful of bases and districts.