Moralizing rhetoric also defines groups on the basis of their tactics rather than their goals. However appalled we might be by a group’s actions, our objective should always be to understand our enemies as they do themselves: in this case, a highly organized insurgency with specific strategic objectives.

This last aspect is particularly important because the discourse of “evil” can create a slippery slope in which almost any countermeasures become permissible to stop the advance of the threat. This week, Mr. Kerry tweeted that ISIS “must be destroyed/will be crushed.” America is still extricating itself from the huge costs and reputational damage sustained by more than a decade of foreign wars begun in the name of stamping out “evildoers.”

For this reason, the Obama administration should be very careful about lapsing into language about “destroying” the cancer of ISIS without thinking through, and articulating publicly, exactly what that would mean. The strategic drift produced by this moralistic language is already noticeable, as an air campaign first designed to prevent a humanitarian catastrophe has morphed into an effort to roll back, or even defeat, ISIS.