America's longest war: Pictures from 20 years in Afghanistan

US Army soldiers from the 2nd Platoon, B battery 2-8 field artillery, fire a howitzer artillery piece at Seprwan Ghar forward fire base in Panjwai district, Kandahar province southern Afghanistan on June 12, 2011.

Captain Melvin Cabebe with the US Army's 1-320 Field Artillery Regiment, 101st Airborne Division stands near a burning M-ATV armored vehicle after it struck an improvised explosive device (IED) near Combat Outpost Nolen in the Arghandab Valley north of Kandahar on July 23, 2010.

An Italian soldier (C) from the NATO cries as he carries a casket which contains the body of one of the six Italian soldiers killed in Kabul, during a repatriation ceremony at the Kabul international airport on September 19, 2009.

A Canadian soldier takes a nap after taking part in a search operation for improvised explosive devices (IEDs) in Kandahar on May 21, 2009.

Spanish soldiers carry the casket of a soldier who died in a helicopter crash, at Herat airport in western Afghanistan. Spanish soldiers carry the casket of a soldier who died in a helicopter crash, at Herat airport in western Afghanistan on August 18, 2005.

US Army soldiers carry Sgt. Matt Krumwiede, who was wounded by an improvised explosive device (IED), towards a Blackhawk Medevac helicopter in southern Afghanistan on June 12, 2012.

Smoke and flames rise from fuel trucks after an overnight attack by the Taliban on the outskirts of Kabul on July 5, 2014.

A US Army soldier with Charlie Company, 36th Infantry Regiment, 1st Armored Division blows a bubble with his chewing gum during a mission near Command Outpost Pa'in Kalay in Maiwand District, Kandahar Province on February 3, 2013.

Taliban fighters train with their weapons in an undisclosed location in Afghanistan on July 14, 2009.

A soldier of the German armed forces Bundeswehr observes the area as villagers look at other Bundeswehr soldiers who are performing a recovery operation on a "Fuchs" armoured personnel carrier, during a mission in Kunduz district on April 28, 2009.