Supporters of cleric Muqtada al-Sadr march in front of two large Iraqi flags with the Arabic words "God is great," in the Sadr City district of Baghdad on Thursday. Tens of thousands of Sadr's followers rallied to demand the U.S. military leave Iraq at the end of the year. Hadi Mizban/AP hide caption

itoggle caption Hadi Mizban/AP

Blackwater security contractors are seen in a helicopter in Baghdad in 2007. The company changed its name to Xe in 2009, amid controversy surrounding the killing of 17 Iraqi civilians in 2007. Khalid Mohammed/AP hide caption

itoggle caption Khalid Mohammed/AP

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

Workers excavate remains from a mass grave in the desert of western Anbar province in Iraq on April 14. The mass grave holds the remains of more than 800 people, believed to have been killed during the rule of ousted leader Saddam Hussein. Ali Mashhadani/Reuters/Landov hide caption

itoggle caption Ali Mashhadani/Reuters/Landov

Ahmed Chalabi, shown in May 2010, says Iraq should lead the way toward democratic change in the region. But some say he may be playing a dangerous game. Karim Kadim/AP hide caption

itoggle caption Karim Kadim/AP