Marines detonate a homemade explosive device in the Helmand River valley in Afghanistan in 2009. David Gilkey/NPR hide caption

itoggle caption David Gilkey/NPR

Air conditioners keep tents cool on a U.S. military base in Iraq. The tents have been treated with polyurethane foam to increase energy efficiency. Courtesy Steven Anderson hide caption

itoggle caption Courtesy Steven Anderson

Army Spc. Christopher B. Fishbeck of Buena Park, Calif., poses for a photo on a rooftop during his tour of duty in Iraq. Courtsey of Fishbeck's Family hide caption

itoggle caption Courtsey of Fishbeck's Family

Alex Horton sits against a wall in Baqubah, Iraq, shortly after a firefight. Courtesy of Alex Horton hide caption

itoggle caption Courtesy of Alex Horton

Army Staff Sgt. Darrell Griffin Jr. was killed in March 2007 in Iraq by a sniper. Courtesy of Darrell Griffin Sr. hide caption

itoggle caption Courtesy of Darrell Griffin Sr.

1st Lt. Fred Simpson considers a computerized mental exercise designed to improve focus and memory. Simpson was knocked unconscious by a grenade blast in Afghanistan and now receives treatment at Walter Reed Army Medical Center in Washington. Maggie Starbard/NPR hide caption

itoggle caption Maggie Starbard/NPR

Soldiers with traumatic brain injury clear improvised explosives on a mock mission while taking fire from pretend insurgents armed with paintball guns. Blake Farmer/Nashville Public Radio hide caption

itoggle caption Blake Farmer/Nashville Public Radio