BELGRADE, Serbia (AP) — Tensions between Serbia and Kosovo flared anew during the past week after Serbs erected barricades on the main roads in the north of Kosovo, a former Serbian province. They were protesting the arrest of a former Kosovo Serb police officer.

Shots were fired from the barricades, and a Kosovo Albanian police officer was injured. Someone lobbed a stun grenade at a European Union peacekeeping patrol mission. Serbia raised its combat readiness and warned it would not stand by if Serbs in Kosovo are attacked.

Serbian President Aleksandar Vucic has called for deploying Serb troops to northern Kosovo, further fueling fears of a revival of the 1998-99 war in Kosovo that claimed more than 10,000 lives and left over 1 million homeless.

A look at the history between Serbia and Kosovo, and why the latest tensions are a concern for Europe.

WHY ARE SERBIA AND KOSOVO AT ODDS?

Kosovo is a mainly ethnic Albanian territory that declared independence from Serbia in 2008. The Serbian government has refused to recognize Kosovo’s statehood and still considers it part of Serbia, even though it has no formal control there.

Over 100 countries have recognized Kosovo’s independence, including the United States and most Western countries. Russia, China and five European Union nations have sided with Serbia. The deadlock has kept tensions simmering and prevented the Balkan region’s full stabilization following the bloody breakup of former Yugoslavia in the 1990s.

HOW DEEP IS THE CONFLICT?

The dispute over Kosovo is centuries-old. Serbia cherishes the region as the heart of its statehood and religion. Numerous medieval Serb Orthodox Christian monasteries are in Kosovo. Serb nationalists view a 1389 battle against Ottoman Turks there as a symbol of its national struggle.

Kosovo’s majority ethnic Albanians, most of whom are Muslim, view Kosovo as their country and accuse Serbia of occupation and repression. Ethnic Albanian rebels launched a rebellion in 1998 to rid the country of Serbian rule. Belgrade’s brutal response prompted a NATO intervention in 1999, which forced Serbia to pull out and cede control to international peacekeepers.

IS THERE A LINK TO RUSSIA AND THE WAR IN UKRAINE?

Well before Russian tanks and troops rolled into Ukraine in February, Russian President Vladimir Putin cited the breakup of Yugoslavia to justify a possible invasion of a sovereign European country. Putin claimed NATO’s bombardment of Serbia in 1999 and the West’s acceptance of Kosovo as a country created an illegal precedent that shattered international law and order.

Putin’s argument, repeated several times since Russia annexed Crimea from Ukraine in 2014, followed this reasoning: If ex-Yugoslav republics and a Serbian province could become independent with Western backing and wars, why shouldn’t Ukraine’s strategic Black Sea peninsula and the rebel-controlled, majority Russian areas in the country’s east split off with Russian help?

Western officials have vehemently rejected Putin’s argument, saying the NATO intervention in Kosovo was triggered by mass killings and other war crimes committed by Serbian troops against ethnic Albanians.

Although that wasn’t the situation in Ukraine before Russia’s full-scale invasion this year, Putin still relies on what happened in Kosovo as a precedent for sending troops in. He has cited Kosovo and Serbia many times since the invasion.

WHAT IS THE SITUATION IN KOSOVO?

There are constant tensions between Kosovo’s government and ethnic Serb residents who keep close ties with Belgrade. Government attempts to impose more control in the Serb-dominated north are usually met with resistance.

Mitrovica, the main city in northern Kosovo, is effectively divided into an ethnic Albanian part and a Serb-held part, and the two sides rarely mix. There are also smaller Serb-populated enclaves in southern Kosovo. Tens of thousands of Kosovo Serbs live in central Serbia, where they fled together with the withdrawing Serb troops in 1999.

Kosovo is a poor country, with much of its prewar industry not running. Crime and corruption are rampant in both the ethnic Albanian and Serb-controlled areas. Serb’s made up 10% of the population before the war, but now it’s lower.

WHAT HAS BEEN DONE TO RESOLVE THE DISPUTE?

There have been constant international efforts to find common ground between the two former war foes, but no comprehensive agreement has emerged so far. European Union officials have mediated negotiations designed to normalize relations between Serbia and Kosovo since 2012.

The negotiations have led to results in some areas, such as freedom of movement without checkpoints and establishing multi-ethnic police forces in Kosovo. However, the latter broke down when Serbs pulled out of the force a few months ago to protest Pristina’s decision to ban Serbian-issued vehicle license plates and demand their replacement with Kosovo-issued ones.

After international pressure, Kosovo Prime Minister Albin Kurti suspended the decree but that did not bring Serbs back to the Kosovo institutions, including the government, hospitals in the north and the police.

WHO ARE THE MAIN PLAYERS?

Kosovo and Serbia both have nationalist leaders who were active during the era of the 1998-99 war. Kurti, Kosovo’s prime minister, is often accused by international mediators of making moves that trigger unnecessary tensions.

For example, he has rejected the idea of a territory swap between Serbia and Kosovo, an idea his political predecessors were willing to consider to reach a negotiated settlement with Serbia. Vucic, Serbia’s populist president, is a former ultra-nationalist who insists that any solution must involve a compromise if it is to last.

Vucic has acknowledged Serbia’s loss of control over Kosovo and said he has come to terms with that, but also says the country won’t settle unless it gains something.

WHAT HAPPENS NEXT?

International officials still hope Kosovo and Serbia can reach a deal that would allow Kosovo to get a seat in the United Nations without Serbia having to explicitly recognize its statehood. Both nations must normalize ties if they want to advance toward EU membership.

No breakthrough in the EU-mediated negotiations would mean prolonged instability, economic decline and the constant potential for clashes. Any Serbian military intervention in Kosovo would mean a clash with NATO peacekeepers there, and Serbia is unlikely to move in.

But Belgrade controls Kosovo’s north, and Kosovo is unlikely to become a member of the U.N. and a functional state without resolving the dispute with Serbia. Kosovo has announced that it will soon apply to become a candidate for EU membership, something Serbia vehemently opposes.