Families gather at a Kurdish checkpoint after fleeing the violence in Iraq's northern Nineveh province. Photographer: Safin Hamed/AFP/Getty Images
Families gather at a Kurdish checkpoint after fleeing the violence in Iraq's northern Nineveh province. Photographer: Safin Hamed/AFP/Getty Images

A sectarian war has rekindled in Iraq. In mere months or even weeks, the country may well be divided among its warring Sunni, Shiite and Kurdish parts. To which one might reasonably respond: Would that be so bad?

An answer of sorts came today, when Kurdish troops seized the northern city of Kirkuk and Iran sent troops to help forces loyal to Iraq's Shiite-led government halt the advance of Sunni insurgents from the Islamic State of Iraq and the Levant, or ISIS. The uncontrolled breakup of Iraq would lead to a destabilizing regional Shiite-Sunni war, led on both sides by radical Islamists hostile to the West. So, to answer the question: Yes, it would be pretty bad.

And that's the frustrating part. Saying "yes" to this question, being awake to the danger ISIS poses, doesn't necessarily lead to any obvious course of action. What to do about ISIS is a separate question from what ISIS is.

Iraqi Prime Minister Nouri al-Maliki, a Shiite, appears to have turned to Iran because the U.S. has not acted quickly enough. President Barack Obama has yet to respond to Maliki's request for airstrikes, though he listed some of the help Iraq is already receiving (military equipment, intelligence assistance) and said no options have been ruled out -- with the significant exception of ground troops.

Obama's hesitation is justified. The U.S.'s disastrous experience in Iraq over the past decade counsels caution. At the same time, a closer look at ISIS (also known as ISIL) is bracing.

ISIS is the better-funded, better-organized successor to al-Qaeda in Iraq, the group that the U.S. eventually thwarted, with Sunni help, more than half a decade ago. ISIS has demonstrated its viciousness and brutal sectarianism, promising to clean out the "filth" of Iraq's Shiite shrines. And it has declared its intention to build a regional Sunni caliphate, starting in Iraq and Syria. ISIS has had so much success in attracting foreign jihadists to fight that the group's leader, Abu Bakr al Baghdadi, has been able to challenge al-Qaeda for pre-eminence among radical Islamists.

The group is also methodical and ambitious. It has published a 400-page annual report with tables and metrics for its successes in such areas as assassinations; Sunni "apostates" made to repent; Shiites killed; and suicide bombings carried out in Iraq. The number of ISIS suicide attacks rose to 78 in 2013, up from 22 the year before. The number of towns held increased to eight. And with Mosul and Tikrit already under ISIS control, 2014 promises to be another banner year.

ISIS is, in other words, not just a terrorist organization but a military force, whose effectiveness is now being proved. One day, the thousands of radicalized foreigners who have fought for them, including many Europeans and Americans, will go home. It would be foolish to assume they won’t carry on their fight there, and in fact at least one may already have.

It's possible -- likely, even -- that ISIS will inflame animosity and violence in a part of the world that has more than its share of both. It's also possible -- less likely, surely -- that the group is so destabilizing to so many that it will unite, at least temporarily, the factions that divide Iraq and neighboring Syria. Its actions may force Maliki to cooperate with his political opponents.

"Iraq's going to need more help," Obama said today. That much is obvious. The question is what form that help should take: where it comes from, what it consists of and, crucially, who else besides the U.S. is delivering it.

To contact the senior editor responsible for Bloomberg View's editorials: David Shipley at davidshipley@bloomberg.net.