For almost three decades, the most dangerous unresolved conflict in wider Europe has lain in the mountains of the South Caucasus, in a small territory known as Nagorno-Karabakh. In the late 1980s, the region confounded the last Soviet leader, Mikhail S. Gorbachev. In the early 1990s, the conflict there created more than a million refugees and killed around 20,000 people. In 1994, after Armenia defeated Azerbaijan in a fight over the territory, the two countries signed a truce — but no peace agreement.

Nagorno-Karabakh erupted again last weekend. It seems one of the players — most likely Azerbaijan — decided to change the facts on the ground. Dozens of soldiers from both sides were killed before a cease-fire was proclaimed on Tuesday. It could fall apart at any moment. The situation is volatile, and there is a danger that the conflict could escalate further unless the international community stops it.

A new all-out Nagorno-Karabakh conflict is the stuff of nightmares. Given the sophisticated weaponry both sides now possess, tens of thousands of young men would most likely lose their lives. Russia and Turkey, already at loggerheads and with military obligations to Armenia and Azerbaijan, respectively, could be sucked into a proxy war. Fighting in the area would also destabilize Georgia, Iran and the Russian North Caucasus. Oil and gas pipeline routes from the Caspian Sea could be threatened, too.

At the heart of the issue is the status of the Armenian-majority highland enclave of Nagorno-Karabakh, which was part of Soviet Azerbaijan. As the Soviet Union crumbled, ethnic Armenians in the territory campaigned to join Armenia. This became a full-scale war, and the Armenians have maintained control of the territory since.