ARLINGTON, VA - SEPTEMBER 11: U.S. President Barack Obama (L), speaks while flanked by Secretary of Defense Ashton Carter, during a ceremony to mark the 15th anniversary of the 9/11 terrorists attacks at the Pentagon Memorial September 11, 2016 in front of the Pentagon in Arlington, Virginia. This year marks the 15th anniversary of the September 11th terrorist attacks that killed nearly 3,000 people at the World Trade Center, Pentagon and on Flight 93. (Photo by Mark Wilson/Getty Images)
