American hostage Peter Kassig beheaded by ISIS militants in new video released online . Gerges : But the heroic will of the Kurdish defense forces in Kobani have broken the group 's will -- and most significantly , they have shattered ISIS ' narrative of invincibility . Kassig 's beheading reveals a terror group that is desperate and on the run . Comment: American-led airstrikes are working and ISIS ' leaders are being driven underground . Gerges : Sunni tribes that were loyal to ISIS are now peeling off amid the group 's ongoing brutality .
American hostage Peter Kassig beheaded by ISIS militants in new video released online . Gerges : Kassig 's beheading reveals a terror group that is desperate and on the run . Comment: American-led airstrikes are working and ISIS ' leaders are being driven underground . Gerges : Sunni tribes that were loyal to ISIS are now peeling off amid the group 's ongoing brutality .