Ethnic violence is back in the Balkans. And once again, it has taken the West by surprise.

This time the focal point is northern Kosovo, the region north of the Ibar River. Formally under Kosovar sovereignty, the area is claimed by Serbia and treated by the West as a de facto part of Serbia, where Serb paramilitaries profit from smuggling to the Albanian mafia while enforcing obedience among the area’s overwhelmingly Serb population.

NATO’s Kosovo Force (KFOR) units—which include some U.S. troops—and the EU’s rule of law mission (EULEX) tread gingerly around this reality. Belgrade exploits the paramilitaries to reinforce Serbia’s territorial claims. Officials from Pristina rarely step into this international shadowland. The United States, European Union and Belgrade all regard Pristina as a distant and inconvenient landlord, a silent partner in their tripartite understandings. Kosovo is expected to abide by the status quo and not fuss over the disjunction between Western professions of Kosovar sovereignty (albeit with close ties to Belgrade) and the reality of dominating Serb paramilitaries on the ground.

These arrangements were upended recently when Kosovo’s prime minister—finally, in the view of his countrymen—sent police units to two northern border posts to enforce a trade-policy decision by the Kosovo government. As Kosovar officials have frequently complained to the EU and the United States (to no avail), Serbia freely exports its goods into Kosovo but blockades Kosovar goods headed north. Kosovo’s deputy prime minister publicly warned Belgrade that unless it lifted the blockade within thirty days, Pristina would block Serbian goods entering through the north. On July 25—after informing EULEX of its intentions, according to the prime minister—he sent Kosovar police units to take charge of two northern crossing points along the Serbian border. The action led to a confrontation with armed Serbs, and one Kosovar police officer was killed in the shooting.

What followed were scenes reminiscent of Croatia and Bosnia in the early 1990s: Barricades went up throughout the north and began restraining KFOR movements; Two leading officials from Belgrade responsible for relations with Kosovo crossed into the north to show solidarity with fellow Serbs manning the barricades, underscore Belgrade’s claim over the territory and try to restore the status quo ante by negotiating with the KFOR commander; Serbs threw Molotov cocktails into a KFOR camp and set the two border posts ablaze. KFOR, as part of a compromise solution to tamp down the violence, took over control of the two destroyed border gates.

By Thursday a deceptive calm had returned to the area and the Kosovar forces had withdrawn. But the violence may have created a new reality. With emotions running strong on both sides, positions seem to be hardening. The Kosovars have united behind their prime minister, who condemned an already-unpopular EULEX for doing nothing to aid the police unit.

Another casualty may be the EU-sponsored Serbia-Kosovo bilateral talks on small but practical technical problems. Just weeks ago, the EU proudly announced agreement on three such issues and hoped to build momentum for further limited dialogue. But pictures of the Serbian leader of those talks meeting with paramilitaries at the barricades cast the negotiating process into a new and less benign light. The events also stirred nationalists in Belgrade. In Pristina, Kosovars now see the talks as a sideshow in which their negotiators are used as props by Brussels to help make Serbia more presentable to governments deciding soon on the country’s EU-candidacy status.

The greater damage may be to a fundamental US policy assumption: that it is better to delay grappling with the undiscussed core issue of the status of northern Kosovo—the claim in Belgrade that Kosovo be partitioned along the Ibar River and the equally firm insistence by the Kosovars that the north belongs to Kosovo.

It may well be that the West’s preference of restoring the status quo ante in the north will not be possible, though diplomats in Pristina, Belgrade and Brussels are working assiduously to achieve that. If so, Washington will have to decide whether to reexamine long-held assumptions about keeping final-status negotiations over the north in a diplomatic deep freeze and consider whether events are now forcing its hand. If this were to happen, it would require the United States to play a major role in the present negotiations and the overall Serbia-Kosovo divide.

Clearly Washington would prefer not to take on this set of headaches—far easier to kick the can down the road. Once violence, however, enters an issue, it can outrace the efforts of diplomats to contain it, as we learned in the Balkans in the 1990s.

Washington now faces two broad policy choices: follow the EU in attempting to restore the old situation by coaxing Pristina to accept the status quo ante in the north and convincing both parties return to their limited talks. Or it could try to shape a new reality, either by changing the nature of the talks and focusing on the fundamental question of the future of the north; or by leading the EU in establishing Western control over northern Kosovo and the border with Serbia. Almost certainly, if history is any guide, short-term considerations will prevail over long-term ones.