Just how terrifying is the Sunni Muslim extremist group that has taken over a huge swath of territory in northern Iraq? Here are some clues:
After seizing Iraqs second-largest city, the group, known as the Islamic State of Iraq and Syria, boasted of massacring 1,700 prisoners in cold blood.
ISIS leaders have announced that they intend to assassinate Iraqs 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.
But the groups 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 Easts 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 administrations diplomacy succeeds even moderate Iraqi Sunnis and Shiites.
The danger, as these unlikely alliances see it, isnt that ISIS could take over all of Iraq; the group has fewer than 15,000 fighters, and the areas it hasnt 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 first step, which President Obama has begun, involves nudging Iraqs Shiite prime minister, Nouri al-Maliki, to broaden his government to include more participation by Sunni leaders.
Iraqs 2005 constitution calls for considerable decentralization of government authority among the countrys regions, and it was designed to encourage power-sharing among the countrys three major groups, the Shiites, Sunnis and Kurds. But al-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 al-Maliki reverse those policies as the price of significant military aid from the United States.
That political effort, officials say, rather than any proposal for joint military action was at the heart of the talks U.S. officials had last week with Iran. The United States hopes the Tehran government will put similar pressure on al-Maliki.
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 Turkeys 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 Iraqs divided south.
Theres one more pair of adversaries who arent talking yet but who also might find themselves uncomfortably on the same side: al-Maliki and Saudi Arabias 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 al-Maliki in Iraq.
Obama hoped he was done with Iraq after the last U.S. forces withdrew in 2011. Were 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.