Afghanistan is again under the control of the Taliban, a fundamentalist group that ruled the nation for five years before U.S.-led forces ousted them in 2001. Refugees fleeing the group’s ultraconservative brand of Islam have swelled the population of Kabul, and worries are spreading that the Taliban’s return might encourage Islamist movements elsewhere in Central Asia more than two decades after Osama bin Laden first sought refuge under their protection.

What is happening in Kabul?

Taliban forces entered the capital of Kabul on Aug. 15, effectively ending a 20-year effort by the U.S. and other Western nations to remodel Afghanistan as a modern democracy. They were buoyed in part by an agreement with the Trump administration in February last year for U.S. forces to leave the country, with President Biden subsequently setting Aug. 31 as the exit date. With Afghan government forces losing air cover and plagued by desertions, the Taliban quickly expanded their footprint before taking the last remaining cities, including Kabul.

Their arrival in the capital sparked panic. Afghans who worked with Western armed forces or agencies rushed to Hamid Karzai International Airport seeking a way out. Western embassies moved their staff to the airport, which is under U.S. military control. In chaotic scenes there, crowds of Afghans ran alongside military transport planes as they prepared for takeoff, with some people trying to cling to the sides of the aircraft. Afghans and Westerners stranded in Kabul trickled into the airport for evacuation, but entry remained difficult, with Taliban fighters manning checkpoints and no clear system to bring people in.

Why did Afghanistan’s capital fall so quickly?

Afghanistan’s national army and police forces, theoretically numbering 350,000 men and trained and equipped at huge cost by the U.S. and Western allies, were supposed to be a powerful deterrent to the Taliban. They were trained to match the way American forces operate, combining ground operations with air power and using aircraft to resupply far-flung outposts and collect intelligence.

But following Mr. Biden’s withdrawal plan, the U.S. pulled its air support, intelligence and contractors servicing Afghanistan’s planes and helicopters. That meant the Afghan military couldn’t function anymore. In many instances, soldiers simply changed out of the uniforms into civilian clothes. Speaking at the White House on Aug. 16, Mr. Biden said he stands “squarely behind” his decision to withdraw U.S. troops from Afghanistan, though he acknowledged that the Taliban took control far more quickly than he expected. He cast much of the blame on the Afghan military for failing to take up the fight with the insurgents.