Defense Secretary Ash Carter testifies on Capitol Hill in Washington earlier this month. In an interview on CNN Sunday, Carter complained that Iraqi forces lacked "the will to fight" the self-declared Islamic State. Manuel Balce Ceneta/AP hide caption

itoggle caption Manuel Balce Ceneta/AP

Members of the anti-war activist group CodePink interrupt a Senate Armed Services Committee hearing with Defense Secretary Chuck Hagel (left) and Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff. J. Scott Applewhite/AP hide caption

itoggle caption J. Scott Applewhite/AP

Joint Chiefs Chairman Gen. Martin Dempsey speaks during a news conference at the Pentagon in December. Manuel Balce Ceneta/AP hide caption

itoggle caption Manuel Balce Ceneta/AP

Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, talking to U.S. military personnel in Tokyo last April. Kyodo/Landov hide caption

itoggle caption Kyodo/Landov

Gen. Martin E. Dempsey, Chairman of Joint Chiefs of Staff, in November of 2013. Paul J. Richards /AFP/Getty Images hide caption

itoggle caption Paul J. Richards /AFP/Getty Images

Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey, Secretary of State John Kerry, and Defense Secretary Chuck Hagel testify before the Senate Foreign Relations Committee on Tuesday. Mark Wilson/Getty Images hide caption

itoggle caption Mark Wilson/Getty Images

Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey. Win McNamee/Getty Images hide caption

itoggle caption Win McNamee/Getty Images

Aug. 13: At Dover Air Force Base in Delaware, the remains of Marine Lance Cpl. Gregory T. Buckley are carried during a dignified transfer. He was killed in a "green on blue" attack. Mark Wilson/Getty Images hide caption

itoggle caption Mark Wilson/Getty Images