Just how terrifying is the Sunni Muslim extremist group that’s taken over a huge swath of territory in northern Iraq? Here are some clues:

After seizing Iraq’s second-largest city, the group, known as the Islamic State of Iraq and Syria, or ISIS, boasted of massacring 1,700 prisoners in cold blood.

ISIS leaders have announced that they intend to assassinate Iraq’s Shiite Muslim religious leaders and destroy their shrines.

ISIS is so extremist that even al-Qaida expelled the group earlier this year for being too violent against fellow Muslims.

“This is al-Qaida on steroids,” Ryan Crocker, the former U.S. ambassador to Iraq, said this week.

But the group’s brutality and extremism may, in the end, contain the seeds of its destruction.

ISIS is so frightening that it is driving some of the Middle East’s traditional adversaries together in a de facto coalition to stop its advance: the United States and Iran, Turkey and the Kurds, and ― if the Obama administration’s diplomacy succeeds ― even moderate Iraqi Sunnis and Shiites.

The danger, as these unlikely alliances see it, isn’t that ISIS could take over all of Iraq; the group has fewer than 15,000 fighters, and the areas it hasn’t entered ― including Baghdad ― are overwhelmingly Shiite.

Rather, the goal is to block ISIS’ proclaimed goal of touching off a sectarian Sunni-Shiite war across the entire Arab world, beginning with Syria and Iraq.

“The most important challenge is reversing the momentum in Iraq that could tip the country back into civil war,” said Colin Kahl, a former top Pentagon official in Iraq who now teaches at Georgetown University. “There are plenty of actors in the region who share an interest in that not happening.”

The first step, which President Obama has already begun, involves nudging Iraq’s Shiite prime minister, Nouri Maliki, to broaden his government to include more participation by Sunni leaders.

Iraq’s 2005 constitution calls for considerable decentralization of government authority among the country’s regions, and it was designed to encourage power-sharing among the country’s three major groups, the Shiites, Sunnis and Kurds. But Maliki has ignored those provisions, centralizing power in Baghdad and arresting major Sunni leaders ― policies that have driven many Sunnis closer to ISIS, despite its brutality.

Obama has demanded that Maliki reverse those policies as the price of significant military aid from the United States. “In the absence of this type of political effort, short-term military action ... won’t succeed,” Obama said last week.

That political effort, officials say, rather than any proposal for joint military action was at the heart of the talks U.S. officials had this week with Iran. The United States hopes the Tehran government will put similar pressure on Maliki.

“Our interests and Iran’s interest are pretty congruent in Iraq ― not perfectly congruent, but close enough,” Kahl said. “Iran doesn’t want Iraq to be overrun by ISIS, it doesn’t want Iraq to slide back into civil war, and it doesn’t want Iraq to be a failed state.”

Meanwhile, in northern Iraq, the crisis has helped cement ties between an increasingly autonomous Kurdish area and neighboring Turkey after years of tension.

Once, Turkey feared the growth of an independent Kurdistan in Iraq out of fear it would fuel separatist sentiments among Turkey’s large Kurdish minority. But in recent years, the Iraqi Kurdish regional government of Massoud Barzani has struck an economic alliance with Turkey, routing new oil pipelines across the northern border.

Now, with the rise of ISIS, Iraqi Kurdistan is acting as a kind of security buffer between Turkey and Iraq’s divided south.

There’s one more pair of adversaries who aren’t talking yet but who also might find themselves uncomfortably on the same side: Maliki and Saudi Arabia’s King Abdullah. The men are said to loathe each other; Abdullah considers the Iraqi prime minister little more than an Iranian puppet.

But in May, Saudi officials announced that they had discovered a terrorist cell in Saudi Arabia that was planning anti-government attacks, and they said it was linked to ISIS, which has been partly funded by Saudi donors. The dilemma for Abdullah is deciding whom he fears more: ISIS in Saudi Arabia or Maliki in Iraq.

“The Saudis aren’t going to help Maliki, but they can help by not making things worse,” Kahl said. “And they can play a positive role by encouraging the Sunni tribes in Iraq to respond if Maliki reaches out to them in a constructive way.”

Obama hoped he was done with Iraq after the last U.S. forces withdrew in 2011. “We’re leaving behind a sovereign, stable and self-reliant Iraq,” he said at the time.

But the prospect of a terrorist-ruled mini-state sprawling across much of Syria and Iraq gives him little choice. It will take deft diplomacy as well as military aid to keep Iraq from sliding back into civil war. Much as most Americans, like the president, would prefer to stay out, the only country capable of providing both is the United States.

By Doyle McManus

Doyle McManus is a columnist for the Los Angeles Times. Readers may send him email at doyle.mcmanus@latimes.com. ― Ed.

(Los Angeles Times)

(MCT Information Services)