Atlanta -- AS the Iraqis turn their focus from holding elections to writing a constitution, the make-or-break issue for their nation may be the city of Kirkuk. Situated next to Iraq's northern oil fields, Kirkuk is a setting for all the ethnic-sectarian conflicts that are the historic reality of Iraq - Muslim against Christian, Sunni against Shiite, Kurd against Arab. It is also home to the Turkmens, who are the ethnic cousins of the Turks and look to a willing Turkey as their protector. In their fierce competition for the right to claim Kirkuk, the Turkmens and the Kurds threaten to turn Iraqi internal politics into a regional conflict.

On a visit to Kirkuk last fall, I talked to both Turkmens and Kurds, and it was immediately obvious that both groups have a passion and feeling of possession toward the city, with its impressive citadel built on an ancient tell. . Kirkuk is the center of the Turkmen population in Iraq, while for Kurds, the city is a touchstone of their identity.

Each group employs demographics to back up its claim to the city. The last official Iraqi census, in 1957, listed 40 percent of Kirkuk's population as Turkmen and 35 percent as Kurdish; the rest were Arabs, Assyrians, Armenians and others. Today, the population is roughly 850,000; based on unofficial estimates, the number of Arabs has significantly increased, and the percentages of the Turkmens and Kurds are probably reversed.

When the American invasion of Iraq began in March 2003, Kurdish militias advanced southward from the Kurdish autonomous zone established in the northern third of Iraq in 1991 and entered Kirkuk. Since then the Kurds have used their position as American allies to bring in Kurdish families and thus bolster their demand that Kirkuk be incorporated in the Kurds' autonomous zone.

Their reason is emotional but also economic: Kirkuk is the key to oil fields that represent 40 percent of Iraq's proven petroleum reserves. At the least, those fields constitute an enormous bargaining chip in the negotiations over the future Iraqi government; at most they provide the economic base for a future Kurdish state.

The Kurds' numbers, and their determination to lay claim to Kirkuk, have stoked the already intense hostilities between the Kurds and Arabs that date to the late 1980's, when Saddam Hussein pushed many Kurds out of the city and replaced them with Arabs. But it is the contest for Kirkuk being waged between the Kurds and Turkmens that is the far more serious problem for the United States because the only card the Turkmens of Kirkuk have to play against the Kurds is Turkey. It is a card Ankara is willing to allow them to put on the table.

Turkey holds its own claim to Kirkuk. Unlike the Ottoman territories that were ceded to Iraq in the agreements that came at the end of World War I, Kirkuk was taken from Turkey as a result of the 1923 Lausanne Treaty. Turkish nationalists still regard it as historically part of Turkey. Ankara also asserts guardianship over the Turkmen ethnic minority in northern Iraq. But those are more emotional than political issues. What is mainly driving Turkey's interest in Kirkuk is the long-term problem of Turkey's own rebellious Kurdish minority, which is 20 percent of its population.

Since 1999, Turkish Kurds have attacked Turkey from bases in northern Iraq, in the Kurdish autonomous region. To Turkey's frustration, Iraqi Kurd officials turn a blind eye to their Turkish Kurd cousins' activities, while the Americans have been reluctant to move against the bases for fear of damaging their relationship with the Iraqi Kurds. The Turkish military has taken matters into its own hands by crossing the Iraqi border on occasion to battle the rebels.

But more ominous for American efforts to stabilize Iraq are Turkish fears that Baghdad will be forced to allow the Kurds to make Kirkuk part of their autonomous zone. For Ankara, this would constitute excessive Kurdish autonomy, its red line in Iraq.

The Turkish military has repeatedly warned Iraqi Kurds against changing Kirkuk's demographics. Although it acknowledges that the future of Kirkuk is an internal issue for Iraq, the military insists that the inclusion of the city into the Kurdish autonomous zone is a question in which it intends to play a part. To underline the point, the military makes no effort to hide its plans to send troops if needed to thwart the Kurds' claim to Kirkuk.

Military intervention in northern Iraq is diplomatically risky for Turkey. Having just secured Europe's agreement to open talks on membership in the European Union, Ankara will move with caution. Yet Turkey may well see preventing the emergence of a potentially oil-rich Kurdish political entity on its borders as worth the risk. And Europe may regard keeping the Iraqi Kurds within the boundaries of Iraq, thus promoting stability in the Persian Gulf and in oil markets, as more important than keeping Turkey out of Iraq.

Although publicly circumspect, Washington sees Turkish military involvement as a looming possibility on the complex political landscape of Iraq. Washington has quietly said that the Kurds will not be allowed to take control of Kirkuk. American military bases in northern Iraq are discreetly being reinforced. And the First Infantry Division that has been in charge of Kirkuk for the last year has balanced the rights of the Turkmens and Arabs against those of the Kurds.

So Washington recognizes that the Kurds, further emboldened by their anticipated numbers in the new Iraqi Parliament, could precipitate a crisis over Kirkuk. The question is whether the United States or the non-Kurdish members of the new Iraqi government can hold the Kurds in check - a difficult task considering the fervor, especially among younger Kurds, for an eventual Kurdish state.

This is one of the complications of the Iraqi election that the Bush administration has hailed as such a success. If the Kurds try to change the status of Kirkuk, the United States may find itself forced to turn its military power on them. But if America does nothing to hold Kirkuk, it may well find itself in another crisis. Only this one would not be confined to Iraq.

By SANDRA MACKEY
The New York Times

Sandra Mackey is the author of "The Reckoning: Iraq and the Legacy of Saddam Hussein."