The ethnic fault line that bisects northern Iraq, imperfectly dividing Arabs and Kurds, might as well have been geological: the temblors it occasionally produces are as destructive as anything measured on the Richter scale. Yet, since 2003 at least, things have been surprisingly quiet along this line, even if the local population is frequently roused by disquieting rumblings. Arguably, relations between Arabs and Kurds—and between their political capitals, Baghdad and Erbil—have been better than they have been since the turmoil that attended the Ottoman Empire’s disintegration almost a century ago. Subsequent decades witnessed an endless cycle of repression, upheaval and uneasy accommodation, followed once again by repression.

One reason for this unlikely turn of events may be that the United States, for all its post-2003 blunders and missteps in the rest of Iraq, found a way to keep things relatively calm in the north. In 2003, it created a local government in Kirkuk that, while pleasing no single ethnic group, displeased everyone somewhat equally (though for different reasons), and thus focused people’s ire on Washington rather than on each other. Subsequent elections allowed the Kurds to gain political prominence, but the U.S. presence and the area’s multi-ethnic character encouraged a degree of pluralist politics and power sharing—even if Kirkuk’s Arabs and Turkmen chronically grumble about Kurdish domination. Moreover, in late 2009, in response to violent incidents that held the threat of military escalation, the U.S. military created joint Arab-Kurd checkpoints and patrols along the tenuous fault line, often referred to as the “trigger line” because of its potential to set off a cataclysm.

In another month, however, U.S. troops will have fully withdrawn from Iraq. Soldiers embedded with the joint patrols and checkpoints have already pulled out. The only U.S. remnant in the north will be a Kirkuk consulate staffed by diplomats and a small number of military officers operating under the Baghdad Embassy’s Office for Security Cooperation. They will continue to be part of the Joint Coordination Center in Kirkuk, which monitors the Arab-Kurdish peacekeeping effort. Whether this sharp reduction in military personnel will trigger an outbreak of violence is the question all Kirkukis, and their friends abroad, now ask themselves.

The United States has been a better friend to the Kurds than to any other group in Iraq thanks to their loyal support since the invasion, and the Kurds have reciprocated. Yet their leaders are wary of superpowers. The Kurds have never had a protector as mighty as the United States, but they have learned from previous short-lived alliances not to trust any outside powers, who inevitably yoke the relatively weaker Kurds to the pursuit of their own geostrategic interests. Recall Henry Kissinger and his demurral that Washington was not engaged in missionary work after the United States left the Kurds to their fate in 1975, having first led them to believe they were safe to rise up against the regime in Baghdad. Recall also the aftermath of the 1991 Gulf War, when President George H. W. Bush encouraged Iraqis to divest themselves of that same regime, then declined to intervene when Saddam turned his tanks, mortars and helicopter gunships on them.

For this reason, Kurdish leaders, taking advantage of the U.S. invasion, spent the next eight years diversifying their foreign sponsors and roping them in with promises of riches—access to Kurdistan’s hydrocarbon wealth. The aim was future protection when needed.

The government of Nuri al-Maliki has not been able to prevent the Kurds’ creeping control over an area the previous regime felt so strongly about that it engaged in wholesale Arabization, dispossessing Kurds and Turkmen alike and systematically killing ten of thousands of rural Kurds in the 1988 Anfal counterinsurgency campaign. At most, Baghdad has only slowed the Kurds’ political advance and drawn a physical line, entrenching its forces along the length of it. But even that line is no longer the Green Line, the Kurds’ pre-2003 border, but rather a new de facto “trigger line” boundary to what has become a significantly enlarged and enriched Kurdistan.

The latest flare-up took place earlier this month. Government troops took control of Kirkuk’s military airfield from U.S. forces over the objections of the Kurdish governor and his supporters, who have long wanted to turn it into a civilian airport. As in the past, the embassy intervened, and a compromise was reached, keeping in place the current shaky balance of power: government troops will stay in exchange for Maliki’s promise that the airfield will be turned to civilian use one day.

A declining U.S. military presence will diminish Washington’s ability to dampen passions when one side or the other crosses the line—as the Kurds did when they deployed military forces to Kirkuk city in February 2011. Neither Baghdad nor Erbil will likely pause if and when it sees its adversary advancing on the trigger line. Yet U.S. influence will be relatively more effective in restraining the Kurds, as Washington has undercut its ability to pressure Baghdad by its own obvious interest in keeping Iraq tightly bound within a strategic security framework facing Iran. For the Kurds, this means they need to make their diversification efforts work by seeking solace with neighboring Turkey, an erstwhile adversary with which they have kissed and made up, as well as the home governments of many companies that have descended on Kurdistan to feast upon its oil and gas wealth.

Kurdish leaders expect their new patrons to provide protection and hopefully also diplomatic support for an independence bid they are sure to make one day. Turkey’s role in this scheme is ambiguous. A rising power, it has every interest in buying or transporting Kurdistan’s oil and gas but also, somewhat contradictorily, in keeping the Kurds weak, lest they give their ethnic brethren in southeastern Turkey ideas about autonomy—or, more ominously, the now-latent notion of forging a Greater Kurdistan. If Iraqi Kurds are to achieve statehood, it would have to be, in Ankara’s eyes, under Turkey’s strict control.

But in the meantime, Turkey needs the Kurds more and more because its influence in Baghdad has waned after it bet on the wrong horse in the 2010 parliamentary elections. Ankara invested heavily in Ayad Allawi’s secular Iraqiya list as a barrier against Iranian influence, but Maliki prolonged his tenure as prime minister with help from Iran and Syria. Maliki was not amused by Turkey’s manipulations, and relations have suffered as a result.

Attention is focused now on Damascus, where the regime’s collapse would reshape alliances throughout the region. Iran’s star would decline, and so would Maliki’s. But the Kurds would gain relative strength and thus might sniff the chance to push ahead in Kirkuk. Would Maliki still be able to curb their advance? And, if so, at what cost? Should violence break out along the trigger line as it shifts, it could be the birth pangs of a nascent, Kurdish-run, energy-rich, Turkish vassal state in the north that would enjoy significant international support.

Joost R. Hiltermann is deputy program director for the Middle East and North Africa at the International Crisis Group.