"The report, due to be published on Monday, says the greatest danger to the international community are groups who share the IS ideology but are currently being ignored - they number about 100,000 fighters.
"Current Western efforts to define 'moderate' and 'extremist' rebels are bound to fail, because the groups themselves rarely make the distinction, the centre says.
"Some 60% of Syria's major rebel groups are Islamist extremists, and many of the groups share the same aims, the study finds.
"Fewer than a quarter of the rebels surveyed were not ideological, and many were willing to fight alongside extremists and would probably accept an Islamist political settlement to the civil war."
In concrete terms, what the think-tank is saying is that if the Islamic State is defeated there are "at least 65,000 fighters belonging to other Salafi-jihadi groups ready to take its place."
Via the Guardian, the report says: "While military efforts against ISIS are necessary, policy makers must recognize that its defeat will not end the threat of Salafi-jihadism unless it is accompanied by an intellectual and theological defeat of the pernicious ideology that drives it."
Correction: Dec. 21, 2015
An earlier version of this post said that 60 percent of rebel fighters held the views of the Islamic State. The study actually found a third held that view and 60 percent held extreme Islamist views.