Less than a month after the 9/11 attacks, American and allied forces began combat operations in Afghanistan, targeting al Qaeda and the Taliban regime that had been giving al Qaeda protection.

Nearly two decades later, the United States withdrew most of its troops from Afghanistan and the Taliban regained control of the country's capital.

US President Joe Biden admitted that the collapse of the Afghan government happened more quickly than his administration had anticipated, but he refused to back away from his decision to end America's longest war.

"I stand squarely behind my decision," Biden said. "After 20 years, I've learned the hard way that there was never a good time to withdraw US forces."

When he announced the withdrawal in April 2021, Biden said diplomatic and humanitarian efforts would continue in Afghanistan and that the United States would support peace efforts between the Afghan government and the Taliban. He said he had determined that a war that had killed some 2,300 troops and cost more than $2 trillion no longer fit within the pressing foreign policy concerns of 2021.

Editor's note: This gallery contains graphic images. Viewer discretion is advised.

Thousands of Taliban supporters rally in Quetta, Pakistan, near the Afghan border, on October 1, 2001.
Thousands of Taliban supporters rally in Quetta, Pakistan, near the Afghan border, on October 1, 2001.
Paula Bronstein/Getty Images
Al Qaeda leader Osama bin Laden is seen at an undisclosed location in this television image broadcast on October 7, 2001. Bin Laden praised God for the September 11 attacks and swore America "will never dream of security" until "the infidel's armies leave the land of Muhammad."
Al Qaeda leader Osama bin Laden is seen at an undisclosed location in this television image broadcast on October 7, 2001. Bin Laden praised God for the September 11 attacks and swore America "will never dream of security" until "the infidel's armies leave the land of Muhammad."
Al Jazeera TV/Getty Images
A Tomahawk cruise missile is launched from a US ship in the Arabian Sea on October 7, 2001. American and British forces began airstrikes in Afghanistan, targeting al Qaeda and the Taliban regime that had been giving al Qaeda protection.
A Tomahawk cruise missile is launched from a US ship in the Arabian Sea on October 7, 2001. American and British forces began airstrikes in Afghanistan, targeting al Qaeda and the Taliban regime that had been giving al Qaeda protection.
Ruben Sprich/Reuters
Members of the Afghan Northern Alliance, an anti-Taliban group, kill a wounded Taliban fighter they found while advancing toward Kabul, Afghanistan, in November 2001. US airstrikes and Northern Alliance ground attacks led to the fall of Kabul that month.
Members of the Afghan Northern Alliance, an anti-Taliban group, kill a wounded Taliban fighter they found while advancing toward Kabul, Afghanistan, in November 2001. US airstrikes and Northern Alliance ground attacks led to the fall of Kabul that month.
Tyler Hicks/The New York Times/Redux
An Afghan Northern Alliance fighter bursts into laughter as US planes strike a Taliban position near Tora Bora, Afghanistan, in December 2001. Afghan militia leaders declared victory in the battle of Tora Bora and claimed to have captured al Qaeda's last base.
An Afghan Northern Alliance fighter bursts into laughter as US planes strike a Taliban position near Tora Bora, Afghanistan, in December 2001. Afghan militia leaders declared victory in the battle of Tora Bora and claimed to have captured al Qaeda's last base.
David Guttenfelder/AP
Renae Chapman holds her 2-year-old daughter, Amanda, during the funeral service for her husband, Army Sgt. 1st Class Nathan R. Chapman, in Fort Lewis, Washington, in January 2002. He was the first US soldier to be killed by enemy fire during the war in Afghanistan.
Renae Chapman holds her 2-year-old daughter, Amanda, during the funeral service for her husband, Army Sgt. 1st Class Nathan R. Chapman, in Fort Lewis, Washington, in January 2002. He was the first US soldier to be killed by enemy fire during the war in Afghanistan.
Russ Carmack/AFP/Getty Images
Mohboba, 7, stands near a bullet-ridden wall in Kabul as she waits to be seen at a health clinic in March 2002. She had a skin ailment that plagued many <a href="https://www.cnn.com/2016/05/31/middleeast/cnnphotos-afghanistan-between-hope-and-fear" target="_blank">poverty-stricken children in Afghanistan.</a>
Mohboba, 7, stands near a bullet-ridden wall in Kabul as she waits to be seen at a health clinic in March 2002. She had a skin ailment that plagued many poverty-stricken children in Afghanistan.
Paula Bronstein/Getty Images
US soldier Jorge Avino tallies the number of people that his mortar team had killed while fighting in Afghanistan in March 2002.
US soldier Jorge Avino tallies the number of people that his mortar team had killed while fighting in Afghanistan in March 2002.
Joe Raedle/Getty Images
A man and his son watch US soldiers prepare to sweep their home in southeastern Afghanistan in November 2002.
A man and his son watch US soldiers prepare to sweep their home in southeastern Afghanistan in November 2002.
Scott Nelson/Getty Images
Women wait in line to be treated at a health clinic in Kalakan, Afghanistan, in February 2003.
Women wait in line to be treated at a health clinic in Kalakan, Afghanistan, in February 2003.
Paula Bronstein/Getty Images
Mohammaed Mahdi, who lost his foot in a mine explosion, waits for a Red Cross doctor at his home in Kabul in August 2004. This photo was taken by Associated Press photographer Emilio Morenatti, who five years later lost part of his leg when the armored vehicle he was in hit a roadside bomb.
Mohammaed Mahdi, who lost his foot in a mine explosion, waits for a Red Cross doctor at his home in Kabul in August 2004. This photo was taken by Associated Press photographer Emilio Morenatti, who five years later lost part of his leg when the armored vehicle he was in hit a roadside bomb.
Emilio Morenatti/AP
Afghans in Kabul line up to vote in the country's first democratic election in October 2004. Hamid Karzai was sworn in as President in December of that year.
Afghans in Kabul line up to vote in the country's first democratic election in October 2004. Hamid Karzai was sworn in as President in December of that year.
Tyler Hicks/The New York Times/Redux
An Afghan soldier provides security at the site where a US helicopter crashed near Ghazni, Afghanistan, in April 2005. At least 16 people were killed.
An Afghan soldier provides security at the site where a US helicopter crashed near Ghazni, Afghanistan, in April 2005. At least 16 people were killed.
Tomas Munita/AP
US President George W. Bush attends a news conference with Afghan President Hamid Karzai at the Presidential Palace in Kabul in March 2006. It was Bush's first visit to Afghanistan.
US President George W. Bush attends a news conference with Afghan President Hamid Karzai at the Presidential Palace in Kabul in March 2006. It was Bush's first visit to Afghanistan.
Ahmad Masood/Reuters
Girls at the Bibi Mahroo High School raise their hands during an English class in Kabul in November 2006. After the fall of the Taliban, millions of Afghan girls <a href="https://www.cnn.com/2012/09/26/world/asia/cnnheroes-afghan-schoolgirls" target="_blank">were able to attend school</a> and get the education that their mothers couldn't.
Girls at the Bibi Mahroo High School raise their hands during an English class in Kabul in November 2006. After the fall of the Taliban, millions of Afghan girls were able to attend school and get the education that their mothers couldn't.
Paula Bronstein/Getty Images
British Marines take cover during an anti-Taliban operation near Kajaki, Afghanistan, in March 2007. Many other countries also deployed troops to the country.
British Marines take cover during an anti-Taliban operation near Kajaki, Afghanistan, in March 2007. Many other countries also deployed troops to the country.
John Moore/Getty Images
Supplies are dropped to US troops in Afghanistan's Ghazni province in May 2007.
Supplies are dropped to US troops in Afghanistan's Ghazni province in May 2007.
Nicolas Asfouri/AFP/Getty Images
Afghan students recite Islamic prayers at an outdoor classroom in the remote Wakhan Corridor in September 2007.
Afghan students recite Islamic prayers at an outdoor classroom in the remote Wakhan Corridor in September 2007.
Paula Bronstein/Getty Images
US Army Spc. Brandon Olson sinks onto a bunker embankment in Afghanistan's Korengal Valley in September 2007. The Korengal Valley was the site of some of the deadliest combat in the region.
US Army Spc. Brandon Olson sinks onto a bunker embankment in Afghanistan's Korengal Valley in September 2007. The Korengal Valley was the site of some of the deadliest combat in the region.
Tim Hetherington/Imperial War Museums
US Marine Sgt. Nicholas Bender launches a Raven surveillance drone near the remote village of Baqwa, Afghanistan, in March 2009.
US Marine Sgt. Nicholas Bender launches a Raven surveillance drone near the remote village of Baqwa, Afghanistan, in March 2009.
John Moore/Getty Images
US soldiers take defensive positions after receiving fire from Taliban positions in Afghanistan's Korengal Valley in May 2009. Army Spc. Zachary Boyd was still in his "I love NY" boxers because he rushed from his sleeping quarters to join his fellow platoon members. 
US soldiers take defensive positions after receiving fire from Taliban positions in Afghanistan's Korengal Valley in May 2009. Army Spc. Zachary Boyd was still in his "I love NY" boxers because he rushed from his sleeping quarters to join his fellow platoon members. 
David Guttenfelder/AP
US soldiers shield their eyes from the rotor wash of a Chinook helicopter as they are picked up from a mission in Afghanistan's Paktika province in October 2009.
US soldiers shield their eyes from the rotor wash of a Chinook helicopter as they are picked up from a mission in Afghanistan's Paktika province in October 2009.
Chris Hondros/Getty Images
Children watch a Canadian soldier conducting a dusk patrol in Kandahar, Afghanistan, in October 2009.
Children watch a Canadian soldier conducting a dusk patrol in Kandahar, Afghanistan, in October 2009.
Chris Hondros/Getty Images
US soldiers fire mortars from a base in Afghanistan's Kunar province in October 2009.
US soldiers fire mortars from a base in Afghanistan's Kunar province in October 2009.
David Guttenfelder/AP
Troops rest at an airfield in Afghanistan's Helmand province in February 2010.
Troops rest at an airfield in Afghanistan's Helmand province in February 2010.
Tyler Hicks/The New York Times/Redux
Afghan soldiers rush a wounded police officer to an American helicopter in Afghanistan's Kunar province in March 2010.
Afghan soldiers rush a wounded police officer to an American helicopter in Afghanistan's Kunar province in March 2010.
Moises Saman/The New York Times/Redux
Village elders meet in Marja, Afghanistan, in March 2010.
Village elders meet in Marja, Afghanistan, in March 2010.
Moises Saman/The New York Times/Redux