Serbia considers Kosovo part of its territory. Kosovo considers itself an independent state.
It's been 20 years since the former Yugoslav territories were at the centre of one of Europe's most violent conflicts, which killed more than 13,000 people.
Yet the countries are still at odds.
But why are the relations so tumultuous? Euronews explores the history of the Balkan states and how they're still embroiled in a debate over sovereignty.
Kosovo is a small, landlocked nation slightly larger than Cyprus that has a population of about two million people. The majority of Kosovo's population is ethnically Albanian and Muslim.
That's because Serbia first lost the Kosovo region to the Ottoman Turks back in the Middle Ages and only regained the territory in the early 20th century.
The small population of ethnic Serbs live in the north of the territory close to the border between the region and the rest of Serbia.
In the aftermath of World War II, Kosovo largely enjoyed autonomy as a region within Serbia.
That changed drastically in 1989 when Serbian leader Slobodan Milošević brought Kosovo directly under the Serbian government in Belgrade and the dissolution of Yugoslavia brought war to the area.
Following in the footsteps of Slovenia and Croatia, Kosovo declared independence from the Socialist Yugoslav Federation but did not get very far. Few countries recognised the state and, meanwhile, violence had broken out in Bosnia where Muslims were driven out of their homes in what became known as "ethnic cleansing".
By 1996, a paramilitary group had formed. The Kosovo Liberation Army (KLA) began armed conflict in a campaign against Milošević.
Serbian special police killed over 50 members of a KLA leader's family in early 1998 and violence only escalated as Serbian police executed ethnic Albanians in a supposed war on terrorism. The KLA responded with its own campaign of violence.
"We are not going to stand by and watch the Serbian authorities do in Kosovo what they can no longer get away with doing in Bosnia," US Secretary of State Madeleine Albright reportedly said.
The international community condemned the violence.
The UN banned arms sales to Serbia and demanded a ceasefire in 1998 as NATO began planning an intervention. Then a turning point: Serbian special police killed 45 ethnic Albanians in the so-called "Račak Massacre" in 1999.
NATO then launched a 77-day air campaign against Serbia in which some civilians were killed. By the time the Serbians left Kosovo and the KLA demilitarised, over a million people had been displaced and began returning to the country.
Kosovo became a self-governed territory under the United Nations, which was tasked with determining Kosovo's future status.
Talks began in 2006 to determine this status, as the UN hosted 17 rounds of negotiations between Kosovo and Serbia, but the plan for Kosovar independence negotiated by the former Finish President Martti Ahtisaari was rejected by Serbia.
Kosovo declared independence in 2008 but is still not recognised by Serbia as an independent state, though more than 100 countries do.
The EU has led talks between the two countries since 2011 which resulted in several technical agreements including the Brussels agreement which aimed to normalise relations and integrate the Serb minority in northern Kosovo.
But in practice, the agreements were vague, and the process was undone after several setbacks.
One setback was the murder in January 2018 of Oliver Ivanović, an ethnic-Serb politician in northern Kosovo. After he was gunned down, talks on normalising relations were further postponed. Serbia's President Aleksandar Vucic called the killing "an act of terrorism".
Later that year, Vucic and his Kosovar counterpart Hashim Thaci, former wartime enemies, talked about a potential borderline change concerning the four municipalities in the north of the country with a majority Serb population.
In an interview with Euronews in 2019, the British ambassador to Kosovo, Ruairi O'Connell, said there was momentum on both sides to make a deal about the border but that it would take time.
"We don’t need a quick deal, we need the right deal and that needs both sides to come prepared and to talk to their people and get ready to sit down at the table seriously," he said.
It fell apart.
Serbia then blocked Kosovo from joining Interpol, and Kosovo raised tariffs to 100% on the country, increasing smuggling in the northern Serb-majority area.
Kosovo upgraded its security force to a small army in December, increasing tensions between the countries.
Serbia ordered its security forces on alert in May after Kosovo carried out an anti-smuggling operation and arrested officers including ethnic-Serbs in the north of the country. It's an area the government in Pristina still has a hard time bringing under its control.
But Kosovo's prime minister attempted to ease the crisis. "I confirm that the operation is about law enforcement and nothing else. I call Serbs in the north to stay calm and respect the law," Kosovo Prime Minister Ramush Haradinaj said.
But tensions appear to be at an all-time high.