In this photo released by the U.S. Marines and taken in December 2010, Lance Cpl. Dakota Hicks, from Laharpe, Ill., connects a radio battery to a portable solar panel communication system in Sangin District, in Afghanistan.The U.S. military is trying to wean itself off reliance on fossil fuels by employing solar energy and biofuels, among other measures. Gunnery Sgt. William Price Small/AP hide caption

itoggle caption Gunnery Sgt. William Price Small/AP

Gen. David Petraeus in Kandahar, Afghanistan, on July 4, 2011. U.S. Navy Lt. j.g. Haraz N. Ghanbari/Getty Images hide caption

itoggle caption U.S. Navy Lt. j.g. Haraz N. Ghanbari/Getty Images

Staff Sgt. Joshua White (center), Command Sgt. Maj. John Troxell (left) and Brigade Sgt. Maj. Mike Boom (right) observe a joint patrol of U.S. Army and Afghan National Army soldiers and Afghan police in Paktika province, Afghanistan, on Oct. 3. The mountainous border between Afghanistan and Pakistan has become a new front line in the Afghan war. Matt Ford/AP hide caption

itoggle caption Matt Ford/AP

Relatives comfort an Afghan man outside a Kabul hospital where victims of Tuesday's suicide bombing were taken. A suicide bomber struck a crowd of Shiite worshippers marking a holy day in the Afghan capital. Ahmad Jamshid/AP hide caption

itoggle caption Ahmad Jamshid/AP