The ISIS fight: More than guns and bombs

The Islamic State's ability to sustain itself through recruitment, propaganda and financing of its operations have bolstered its ability to survive as the U.S.-led military campaign against it has faltered. But U.S. officials are focusing on changing that dynamic and hoping to alter the military balance as well.

In testimony before Congress this week, Joint Chiefs Chairman Gen. Joe Dunford and John Allen, the outgoing coordinator for the coalition's fight against the extremist group, both said more needs to be done to counter the group's successes in spreading its message and gathering foreign extremists to join its fight.

"To be successful, the coalition's military campaign must reduce [the Islamic State's] territorial control, destroy its warfighting capability and undermine its brand and aura of invincibility," Dunford told the Senate Armed Services Committee on Tuesday.

Foreign fighters are a key source of manpower for the Islamic State. A report released at the end of September by the House Homeland Security Committee estimated that more than 25,000 foreigners were fighting for the group in Syria, including more than 250 U.S. citizens. The United Nations estimates that number has grown to about 30,000 from 104 countries, twice the number as a year ago.