EXECUTIVE SUMMARY: The West should seek the further weakening of Islamic State, but not its destruction. A weak but functioning IS can undermine the appeal of the caliphate among radical Muslims; keep bad actors focused on one another rather than on Western targets; and hamper Iran’s quest for regional hegemony.

US Defense Secretary Ashton Carter recently gathered defense ministers from allied nations to plan what officials hope will be the decisive stage in the campaign to eradicate the Islamic State (IS) organization. This is a strategic mistake.