U.S. Navy Petty Officer 2nd Class Rynard S. Walker wears a gas mask during a chemical, biological, radiological, nuclear hazard scenario as part of Active Shield 2011 at Marine Corps Air Station Iwakuni, Japan, March 2, 2011. Active Shield is a combined anti-terrorism training exercise intended to strengthen cooperation between U.S. and Japanese forces in the Iwakuni-Hiroshima region and prepare Japanese forces to guard U.S. facilities if needed. U.S. Marine Corps photo by Cpl. Andrea M. Olguin/Released)