Shahidullah Shahid : More and more terrorist and insurgent groups are swearing loyalty to ISIS . In 1998 , ISIS has attracted some half dozen groups , he says . Bergen ' perceived success is attracting these recruits , ISIS says .
Peter Bergen : More and more terrorist and insurgent groups are swearing loyalty to ISIS . In past six months , ISIS has attracted about a dozen groups , he says . ISIS ' perceived success is attracting these recruits , Bergen says .