WASHINGTON - Turkey's alarm yesterday at the surge by Kurdish forces into the oil-rich city of Kirkuk in northern Iraq threw a spotlight on the ethnic and religious cauldron that American soldiers and diplomats now must keep from bubbling over.
Iraq's population includes most of the Middle East's religions, ethnic groups and cultures, a diversity that many hope will one day become an example of harmony for a hate-riven region.
But historically this patchwork has pitted rival groups against one another and threatened to pull apart a nation cobbled together by the British Empire at the end of World War I.
Now, Iraq's first burst of freedom threatens to open a season of jockeying for power, revenge and score-settling that might preoccupy the United States as much as providing humanitarian relief and putting together a new government that represents the aspirations of all Iraqis.
"How these groups will live with one another in a postwar environment, and how something can be arranged that satisfies all of them, is a daunting prospect," said Amy Hawthorne of the Carnegie Endowment for International Peace, who specializes in political change in the Arab world.
While trying to quell or prevent internal flare-ups, the United States will also have to ease the anxieties of Iraq's neighbors, who fear the impact of Iraqi turbulence on their populations.
"All of this creates real challenges, but they don't necessarily have to be insurmountable," said Kenneth Pollack of the Brookings Institution's Saban Center for Middle East Policy.
Turkey, which has fought a bloody war against militants in its Kurdish population, has long viewed northern Iraq's Kurds with suspicion, fearing that an independent Kurdistan would generate a push for secession among Turkey's Kurds, who are concentrated in southeast Turkey near the border with Iraq.
The oil-rich cities of Kirkuk and Mosul are of particular concern. Turkish leaders fear that if Kurds were to gain control of those cities, they would have a strong economic base for an independent Kurdistan. Turkey threatened to send troops to the region if Kurds were allowed to take control of Kirkuk and Mosul. Turkey also feels protective toward the sizable, ethnically-related Turkoman minority concentrated in and around the two cities.
Iraq's ethnic makeup includes Arabs, Kurds, Turkomen, Assyrians, Circassians and Azeris. Its population breaks down along religious lines to includes Shiite and Sunni Muslims, Christians and a small number of Jews, and is further subdivided among competing tribes.
Even during the days of the Ottoman Empire, which collapsed in World War I, much of the region that Iraq occupies today was dominated by the Sunni Muslim minority, denying majority Shiites a representative share of power. During his 24-year rule, Saddam Hussein enforced this dominance with brutal repression.
Now that Hussein's yoke has been lifted, looters from Saddam City, an impoverished Baghdad Shiite slum, have emerged to ransack the abandoned homes of regime leaders.
Between Shiites and Sunnis, "there's a potential for score-settling that's been repressed for 80 years," said Pollack.
Regional specialists say there is also a strong potential for revenge against members of the Baath Party that has ruled the country since the late 1960s, or against Hussein's network of supporters and those who became rich thanks to the regime - whatever their religion.
It was apparently in an attempt to avoid such a possibility that Abdul Majid al-Khoei, a moderate and respected Shiite cleric returning from exile in London, met his death yesterday in the Iraqi holy city of Najaf. According to accounts from the scene, al-Khoei had accompanied Haider al-Kadar, a hated official in Saddam's Ministry of Religion, to a Muslim shrine in a gesture of reconciliation. Al-Khoei pulled a gun after an angry crowd turned on al-Kadar, and the crowd killed them both with swords and knives.
The variety of ethnic, religious and tribal tensions that could flare up makes governing Iraq so difficult that many leaders in the surrounding region and beyond accepted the presence of a brutal regime as necessary to hold the country together.
No outside power intervened after Hussein's use of poison gas to quell a Kurdish uprising in 1988, nor when he used attack helicopters to kill tens of thousands of Kurds and Shiites who rose up against his regime after the 1991 gulf war. Nor was there a strong outside reaction later in the 1990s, when Hussein drained the marshes that provided the habitat and livelihood for generations of "marsh Arabs" in southern Iraq, who were hostile to the regime. The full extent of Iraqis' suffering is only now becoming known as torture victims and others feel free to talk for the first time.
The coming months will test whether the United States and Britain, as occupying powers, can hold the country together with a lighter touch, persuading Iraqis to suppress ethnic tensions in pursuit of a freer society.