The beheading of American journalist James Foley by the Islamic State in Iraq and Syria, or ISIS, has rightly provoked global condemnation of the insurgent group and its horrific tactics. Yet it has also led to a disturbing return of the moralistic language once used to describe al-Qaida in the panicked days after the 9/11 attacks.

In an eerie echo of President George W. Bush's description of the global war on terrorism as a campaign against "evil doers," President Obama described ISIS as a "cancer" spreading across the Middle East that had "no place in the 21st Century." Secretary of State John Kerry condemned ISIS as the face of a "savage" and "valueless evil," while Britain's prime minister David Cameron called the group "barbaric."

There is no question that ISIS has committed thousands of grave human rights violations against civilians in Iraq and Syria, and that many of its most gruesome acts, like the execution of Foley, constitute war crimes. Anyone with a conscience is disgusted by their brutality toward not just Foley but thousands of Iraqi and Syrian civilians.

It is natural to want to condemn this organization in harsh language that fully expresses our revulsion over its tactics. Condemning the black-clad, masked militants as purely "evil" is seductive, for it conveys a moral clarity and separates ourselves and our tactics from the enemy and theirs.

But if the war on terror has taught us anything, it is that such moralistic language can blind its users to consequences. Describing a group as "inexplicable" and "nihilistic," as Kerry did, tends to obscure the group's strategic aims and preclude further analysis. Resorting to ritualized rhetoric can be a very costly mistake if it leads one to misunderstand an enemy and to take actions that inadvertently help its cause.

After 9/11, the Bush administration's repeated use of the language of good and evil played directly into the hands of al-Qaida. For years, the jihadis had portrayed themselves as engaged in a war against the Christian West and preached that America intended to invade and occupy Muslim lands. Bush's calls for a crusade against radical Islam, combined with the occupation of Iraq, confirmed that narrative and gave al-Qaida a boost in funding and recruitment that sustained the group for nearly a decade.

Avoiding the gross simplifications that moralistic language entails is particularly important here because ISIS is not simply al-Qaida 2.0. Out of the chaos of Syria's civil war, ISIS has emerged as a sophisticated, battle-hardened insurgent organization with an ambitious and effective plan for seizing territory.

Unlike al-Qaida, whose dreams of forming a caliphate were little more than mysticism and hyperbole, ISIS now occupies large swaths of Syria and Iraq, administering social services and running rudimentary Sharia courts in its claimed Islamic State. It operates less like a revolutionary terrorist movement that wants to overturn the entire political order in the Middle East than a successful insurgent group that wants a seat at that table.

This fundamental difference, as well as disagreements over strategic priorities and tactics, explains why al-Qaida's leadership expelled ISIS in February. Depicting ISIS as another iteration of al-Qaida-style "evil" ignores these distinctions and shifts attention away from ISIS' strategy and sources of revenue and support.

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 counter-measures become permissible to stop the advance of the threat. Recently, 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 "evil doers."

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.

The Obama administration needs to ensure that the just revulsion over Foley's murder and ISIS' other abuses does not lead us down an unplanned path toward open-ended conflict. The language of good and evil may provide a comforting sense of moral clarity, but it rarely, if ever, produces good policy.

Michael J. Boyle, an associate professor of political science at La Salle University in Philadelphia, is the author of Violence After War: Explaining Instability in Post-Conflict States.

Editorial on 08/31/2014