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

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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

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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

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

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

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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

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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

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