The war in Afghanistan was a prolonged armed conflict lasting from 2001 to 2021. It began with an invasion by a United States–led coalition under the name Operation Enduring Freedom in response to the September 11 attacks (9/11) carried out by the Taliban-allied and Afghanistan-based al-Qaeda. The Taliban were expelled from major population centers by American-led forces supporting the anti-Taliban Northern Alliance, thus toppling the Taliban-ruled Islamic Emirate. In 2004, the U.S.-backed Islamic Republic was established, but by then, the Taliban, led by founder Mullah Omar, had reorganized and begun an insurgency against the Afghan government and coalition forces. The conflict ended as the 2021 Taliban offensive reestablished the Islamic Emirate. It was the longest war in United States military history, surpassing the Vietnam War by six months.
Following 9/11, masterminded by al-Qaeda leader Osama bin Laden, American president George W. Bush demanded that the Taliban immediately extradite him to the United States and close down al-Qaeda's camps in Afghanistan; the Taliban refused and demanded evidence of bin Laden's guilt before offering to hand him over to a neutral country. The US dismissed these offers and proceeded with the invasion. After expelling the Taliban and their allies, the American-led coalition remained in Afghanistan, forming the International Security Assistance Force (ISAF)—sanctioned by the United Nations—with the goal of creating a new democratic authority in the country that would prevent the Taliban from returning to power. A new Afghan Interim Administration was established, and international rebuilding efforts were launched. By 2003, the Taliban had reorganized and launched a widespread insurgency against the new Afghan government and coalition forces. Insurgents from the Taliban and other Islamist groups waged asymmetric warfare, fighting with guerrilla warfare in the countryside, suicide attacks against urban targets, and reprisals against perceived Afghan collaborators. By 2007, large parts of Afghanistan had been retaken by the Taliban following a major offensive in the year prior. In response, the coalition sent a major influx of troops for counter-insurgency operations, with a "clear and hold" strategy for villages and towns; this influx peaked in 2011, when roughly 140,000 foreign troops were operating under ISAF command across Afghanistan.
An American covert operation in neighboring Pakistan led to the killing of Osama bin Laden in 2011, and NATO leaders began planning an exit strategy from Afghanistan. In 2014, NATO formally ended ISAF combat operations in Afghanistan and officially transferred full security responsibility to the Afghan government. Unable to eliminate the Taliban through military means, coalition forces (and separately, the Afghan government led by Ashraf Ghani) turned to diplomacy to end the conflict. These efforts culminated in the United States–Taliban deal in 2020, which stipulated the withdrawal of all US troops from Afghanistan by 2021. In exchange, the Taliban pledged to prevent any militant group from staging attacks from Afghan territory against the US and its allies. However, the Afghan government was not a party to the deal and rejected its terms. Coinciding with the withdrawal of troops, the Taliban launched a broad offensive throughout the summer of 2021, successfully reestablishing their control over Afghanistan, including the capital city of Kabul on 15 August. On the same day, Ghani fled the country; the Taliban declared victory and the war was formally brought to a close. By 30 August, the last American military aircraft departed from Afghanistan, ending the protracted American-led military presence in the country.
Overall, the war killed an estimated 176,000–212,000+ people, including 46,319 civilians. In addition, 66,650 people were killed in the related War in North-West Pakistan. While more than 5.7 million former refugees returned to Afghanistan after the 2001 invasion, by the fall of Kabul, 2.6 million Afghans remained refugees, while another 4 million were internally displaced.
This conflict from 2001 to 2021 is referred to as the war in Afghanistan, in order to distinguish it from Afghanistan's various other wars, notably the ongoing Afghan conflict of which it was a part, and the Soviet–Afghan War. From the perspective of the West, the war is divided between 2001 and 2014 (the ISAF mission), when most combat operations were performed by coalition forces, and 2015 to 2021 (the Resolute Support Mission), when the Afghan armed forces did most of the fighting against the Taliban. The war was named Operation Enduring Freedom from 2001 to 2014 and as Operation Freedom's Sentinel from 2015 to 2021 by the US. Alternatively, it has been called the US war in Afghanistan. In Afghanistan itself, the war is known as simply the "war in Afghanistan".
The Taliban emerged from religious students known as the Talib who sought to end warlordism in Afghanistan through stricter adherence to Sharia. In 1996, the Taliban, with military support by Pakistan and financial support from Saudi Arabia, seized Kabul and founded the Islamic Emirate of Afghanistan. The Taliban imposed their fundamentalist Deobandi interpretation of Islam in areas under their control, issuing edicts forbidding women to work outside the home, attend school or to leave their homes unless accompanied by a male relative. According to the United Nations (UN), the Taliban, while trying to consolidate control over northern and western Afghanistan, committed systematic massacres against civilians. UN officials stated that there had been "15 massacres" between 1996 and 2001, many of them targeting Shias and Hazaras.
By 2001, the Taliban controlled as much as 90% of Afghanistan, with the Northern Alliance confined to the country's northeast corner. Fighting alongside Taliban forces were some 28,000–30,000 Pakistanis (usually also Pashtun) and 2,000–3,000 al-Qaeda militants.
The 9/11 Commission in the US found that under the Taliban, al-Qaeda was able to use Afghanistan as a place to train and teach fighters, import weapons, coordinate with other jihadists, and plot terrorist actions. While al-Qaeda maintained its own camps in Afghanistan, it also supported training camps of other organizations. An estimated 10,000 to 20,000 men passed through these facilities before 9/11, most of whom were sent to fight for the Taliban against the Northern Alliance. A smaller number were inducted into al-Qaeda.
After the 1998 US embassy bombings were linked to bin Laden, US president Bill Clinton ordered missile strikes on an al-Qaeda camp in Afghanistan. US officials pressed the Taliban to surrender bin Laden. In 1999, the international community imposed sanctions on the Taliban to motivate their extradition of him. The Taliban rebuffed these demands. Central Intelligence Agency (CIA) Special Activities Division paramilitary teams were active in Afghanistan in the 1990s in clandestine operations to locate and kill or capture bin Laden. These teams planned several operations but did not receive the order to proceed from Clinton. Their efforts built relationships with Afghan leaders that proved essential in the 2001 invasion.
On the morning of 11 September 2001 (9/11), 19 members of al-Qaeda carried out coordinated attacks in the United States by hijacking four commercial airliners. The hijackers intentionally crashed two of them into the World Trade Center in New York City. The center collapsed as a result. The hijackers crashed a third airliner into the Pentagon in Arlington, Virginia. The fourth plane, which had been targeted at Washington, D.C., crashed into a field in Pennsylvania. No one aboard the four flights survived. Total deaths were 2,996, including the 19 hijackers.
Osama bin Laden planned and coordinated the attacks, and the US desire to hold him accountable became the casus belli for invasion. After the invasion was declared, bin Laden sought, successfully, to draw the US into an extended war similar to that fought against the Soviets. The Taliban publicly condemned 9/11. They also greatly underestimated the US' willingness to go to war. The US was mistaken in its belief that the Taliban and al-Qaeda were almost inseparable when, in fact, they had very different goals and leaders.