In yet another addition to Europe’s already brimming plate of worries, the EU has had to step in to mediate a new spat between Serbia and Kosovo that threatens to derail the still-fragile peace in the Balkans. The disagreement comes on the heels of a string of dismaying developments stemming from the region, not only from Serbia and Kosovo but from their neighbors as well.

With the Union facing some of the most significant threats in its history, from a resurgent Russia to a rising China to alarming new policies emanating from the new American administration, the Balkan dispute seems positively trifling at first glance. However, the EU needs to demonstrate strong leadership not only against the great powers, but in its own backyard as well. Otherwise, it runs the risk of casting aside all the progress that has been made in the nearly 20 years since NATO brought the last war in the Balkans to an end.

The latest spike in tensions between the former wartime adversaries came after Serbian authorities sent a train emblazoned with nationalist slogans to northern Kosovo on January 14th. The train was inscribed with the words “Kosovo is Serbia” in 21 languages, including Albanian, and was decorated with images of Serbian Orthodox saints inside the carriages. It would have served as the first direct railway connection between the two states since 1999, when a clash over northern Kosovo degenerated into a violent conflict. Instead, the train has only served as a provocation that has added fuel to long-simmering ethnic tensions, once again calling into question the uneasy peace between the former foes.

Following the incident, EU Foreign Affairs Chief Federica Mogherini called the leaders of both states to Brussels last week as part of an EU-sponsored dialogue meant to calm bilateral relations. Kosovo’s President Hashim Thaci declared that he was “satisfied” with the results of the discussions, while Serbia’s Aleksandar Vucic only conceded that the talks were “difficult” but had to continue. Vucic’s stubborn attitude echoes the aggressive stance of Marko Djuric, head of the Serbian government office for Kosovo, who said he was unaware why anyone would view the images on the train, which he called “part of the world’s cultural heritage,” as a “provocation.”

So while a full-blown war might be off the books for now, the frosty posturing by Serbia, and Kosovo’s sensitivity to such provocations, don’t bode well for the future of peace in the Balkans. After all, the dispute over the train is far from an isolated incident, but part of a string of worrisome developments in the region.

Just last Thursday, European Council President Donald Tusk cautioned against rising “nationalist rhetoric” in the Western Balkans that might throw Montenegro and its neighbors off their track towards integration with NATO and the EU. He spoke after receiving newly-appointed Montenegrin Prime Minister Dusko Markovic, who came into office last October after elections tarnished by allegations of a Russia-backed coup, hacking of media and party websites, and violence at polling stations. Markovic took the place of former PM Milo Djukanovic, who had been at the helm almost constantly for more than two decades. While some might applaud the transfer in power, Djukanovic is all but certain to remain the strongman behind the throne. Indeed, opposition groups have accused him of manipulating the elections and of fabricating evidence of the supposed coup to maintain a grip on power. The claims are not surprising, given Djukanovic’s shady history. In more than two decades as either prime minister or president of Montenegro, he has overseen a “mafia state” marred by widespread corruption, cronyism, and organized crime. He himself has been accused of criminal ties, including connections with a billion-dollar cigarette smuggling operation.

Given the ruling party’s history of governance – or lack thereof – it’s not surprising that the chapters of Judiciary and Fundamental Rights, and Justice, Freedom and Security, are those preventing Montenegro from progressing further in EU accession negotiations.

But Montenegro, Serbia, and Kosovo are far from the only problem children in the region. In Macedonia, the former Prime Minister and conservative leader Nikola Gruevski failed to form a new alliance with the ethnic Albanian party, therefore missing the deadline for forming a government after the December 11th elections. The failure is another blow to a country that has been going through political upheaval for close to a year, after Gruevski stepped down from his post as PM following an EU-brokered agreement to stop a political crisis that stemmed from a corruption scandal.

Meanwhile, in Bosnia, the prospect of an alliance between the country’s nationalist Croat and Serbian leaders is troubling many Bosniaks, who fear that the growing cooperation could stir up renewed ethnic tensions.

Faced with such an array of destabilizing developments rising from the Balkans, the EU must take a firm position of leadership to ensure that the situation does not deteriorate as it did in the 1990s. To be sure, the Union faces an unprecedented level of threats coming from all sides. And given the Balkans’ continued struggles with economic growth, corruption, organized crime, and rule of law, it’s unlikely that any of the candidate states in the region will join the EU before 2025, if ever. But this doesn’t mean that Brussels should turn a blind eye to these seemingly insignificant issues in its backyard. Far from it. If the Union strengthens its borders, and helps candidate states overcome obstacles on the way to membership, it will be all the stronger for it. In this day and age, Europe will need such strength more than ever.