U.S. Army Soldiers, assigned to Delta Company, 3rd Battalion, 75th Ranger Regiment, set up security with gas mask donned at the National Training Center (NTC), Fort Irwin, Calif., Feb. 23, 2015. NTC is a major training area for the U.S. military to fully exercise their Mission Essential Task List that supports the Army's core competencies. (U.S. Army photo by Spc. Nelson Rodriguez/Released)