When Azerbaijan launched a military offensive earlier this month to retake Nagorno-Karabakh, a mountainous region of the South Caucasus, it reignited the flames of a centuries-old dispute. The Azeri victory, which prompted thousands of ethnic Armenian residents to flee the region, is likely the last in a series of tumultuous battles over who can claim the disputed enclave, a question shaped in modern times by the rise and fall of the Soviet Union.

Officially, the 1,700-square-mile territory is part of Azerbaijan and is known by its Russian name, which translates to “mountainous Karabakh.” But to Armenians and the region's Armenian-majority population, it’s been known as the Republic of Artsakh or the Nagorno-Karabakh Republic, a de facto independent state that was outside of Azeri rule since 1988.

Read how Nagorno-Karabakh’s residents grappled with conflict and COVID-19.

For centuries, Muslim Azerbaijanis and Christian Armenians, both of whom call the region home, clashed over who should control it. Russian rule began in 1823, and when the Russian Empire dissolved in 1918, tensions between newly independent Armenia and Azerbaijan reignited. Three years later, Communist-controlled Russia set its sights on the independent states of the Caucasus region and began incorporating them into what would become the Union of Soviet Socialist Republics.

‘I don’t even know if my home still exists.’ Learn how the first Nagorno-Karabakh war displaced more than a million people in the southern Caucasus.

At first, it was decided that Karabakh would be part of the Armenian Soviet Socialist Republic (S.S.R.). Though historians differ on the reasons, the initial incorporation of Karabakh into the Armenian republic is thought to have been a plan to ensure Armenian support of Soviet rule. But the Soviets’ new Commissar of Nationalities, Joseph Stalin, then reversed the decision. In 1923 Nagorno-Karabakh became an autonomous administrative region within the Azerbaijan S.S.R., even though 94 percent of its population at the time was ethnic Armenian. Ethnic Armenians complained that Azerbaijan restricted their autonomy and claimed Azerbaijan discriminated against them, but the Soviet Union was intolerant of ethnic nationalism and ignored a variety of protests against the status quo.

As the Soviet Union disintegrated in the late 1980s, the long-dissatisfied ethnic Armenians of Nagorno-Karabakh petitioned to become part of the Republic of Armenia. Azerbaijan responded by trying to crush the separatists in 1988, and clashes intensified in the region. In 1991, both Azerbaijan and Armenia declared independence from the U.S.S.R., and the regional conflict in Nagorno-Karabakh flared into full-out war.

As a result, more than a million people became refugees, and around 30,000, including civilians, were killed. Both sides engaged in ethnic cleansing during the Nagorno-Karabakh War—the Azerbaijanis against ethnic Armenians, and Armenian forces against ethnic Azeris. Despite the brutal humanitarian toll, negotiations between the sides repeatedly broke down.

In 1994, the newly independent nations of Armenia and Azerbaijan signed the Bishkek Protocol, a ceasefire brokered by Russia that left Nagorno-Karabakh in Azerbaijan. But though the fighting ceased, the two sides could not agree on a peace treaty.

For the last two and a half decades, Armenian and Azerbaijani troops were divided by a contested “line of contact” laid out in the Bishkek Protocol. It became increasingly militarized over the years, and has been called one of the world’s three most militarized borders.

That’s of even greater importance because of the conflicted nations’ powerful allies. Azerbaijan is supported by NATO member Turkey, while Russia has supported Armenia, making the area a potential conflagration zone. While Nagorno-Karabakh is small, the geopolitical stakes are high due to its proximity to strategic oil and gas pipelines, and its location between the powerful regional forces of Russia, Turkey, and Iran.

The separatist government has announced it will dissolve by year's end, and Azerbaijan has promised to guarantee Armenian rights in the region. But most of the region’s 120,000 ethnic Armenians won’t wait to find out if the Azeris will follow through on that pledge. "Ninety-nine point nine percent prefer to leave our historic lands," David Babayan, an adviser to Samvel Shahramanyan, president of the self-styled Republic of Artsakh, recently told Reuters. As the mass exodus continues, the story of Nagorno-Karabakh appears to be coming to a close—still echoing with the fears and humanitarian toll that have plagued it for centuries.