During the nearly 20 years since the United States went to war in Afghanistan, the number of U.S. troops there reached as high as 100,000 and then plummeted after the 2011 killing of al-Qaida leader Osama bin Laden in neighboring Pakistan to around 2,500 today. Now U.S. President Joe Biden has set Sept. 11, 2021 as the new deadline for full U.S. troop withdrawal from Afghanistan.

Paratroopers from Chosen Company of the 3rd Battalion (Airborne), 509th Infantry board a waiting CH-47 Chinook helicopter as they begin a helicopter assault mission at Combat Outpost Herrera in Afghanistan's Paktiya province, July 15, 2012.

U.S. soldier points his rifle after coming under fire in Zharay district in Kandahar province, southern Afghanistan, April 26, 2012.

Members of the U.S. Navy carry a comrade wounded by an explosion to a medevac helicopter in Kandahar province, southern Afghanistan, Oct. 2, 2010.

U.S. Army soldiers from the 2nd Platoon, B battery 28 field artillery, fire a howitzer artillery piece at Seprwan Ghar forward fire base in Panjwai district, Kandahar province, southern Afghanistan, June 12, 2011.

U.S soldiers blow up a roadside bomb set up by Taliban fighters near the town of Walli Was in Paktika province, near the border with Pakistan, Nov. 4, 2012.

Two 500-pound (227-kilogram) bombs explode at dusk on a Taliban fighting position near Blocking Position one above Kamdesh in Nuristan province, Afghanistan, June 11, 2012.

Ryan Lemm, 4, salutes as he is carried by New York Police Department officer John McCrossen as he watches the casket of his father, NYPD officer Joseph Lemm, who was killed on duty in Afghanistan, following funeral services in the Manhattan borough of New York, the U.S., Dec. 30, 2015.

A woman lies down in front of the grave of her brother at Arlington National Cemetery in Virginia, the U.S., March 11, 2013.

A U.S. Marine Humvee kicks up dust as it crosses a group of Marine Light Armoured Vehicles (LAVs) as it leaves on patrol from the Marine base in southern Afghanistan, Dec. 2, 2001.

A rocket fired by U.S. soldiers from Dragon Troop of the 3rd Cavalry Regiment explodes next to a destroyed bus at operating base Gamberi in Laghman province, Afghanistan, Dec. 31, 2014.

U.S. marines fire during a Taliban ambush as they carry out an operation to clear an area in Helmand province, Afghanistan, Oct. 9, 2009.

MATV armored vehicle burns after it struck an improvised explosive device near Combat Outpost Nolen in the Arghandab Valley, north of Kandahar, Afghanistan, July 23, 2010.

A U.S. soldier takes a break during night mission near Honaker Miracle camp at the Pesh valley of Kunar province, Afghanistan, Aug. 12, 2009.

Sgt. William Olas Bee, a U.S. Marine from the 24th Marine Expeditionary Unit, has a close call after Taliban fighters opened fire near Garmsir in Helmand province, Afghanistan, May 18, 2008.

Stryker armored vehicle fires a 120 mm mortar during a night patrol in Kandahar, Afghanistan, April 27, 2010.