Nagorno-Karabakh is a mountainous and heavily forested patch of land that sits inside the territory of ex-Soviet Azerbaijan.
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Under international law, Nagorno-Karabakh is recognized as part of Azerbaijan. But the ethnic Armenians who make up most of the population reject Azeri rule. They have been running their own affairs, with support from Armenia, since Azerbaijan's troops were pushed out in a war in the 1990s.
Long-standing ethnic tensions in the region between Christian Armenians and their mainly Muslim neighbors flared in Nagorno-Karabakh in the late 1980s. Hostilities this year have been the worst since 2016, when intense fighting killed dozens and threatened to escalate into all-out war.
Such a conflict could drag in the big regional powers, Russia and Turkey. Moscow has a defense alliance with Armenia, while Ankara backs its ethnic Turkic kin in Azerbaijan.
In the 1980s, the territory was within the borders of the then-Soviet republic of Azerbaijan, though most decisions were made in Moscow.
As the Soviet Union began to break up, it became apparent that Nagorno-Karabakh would come under the direct rule of the Azeri government. The ethnic Armenians did not accept that.
Sectarian conflict erupted, escalating into war in 1991 between Azerbaijan's troops and Nagorno-Karabakh's ethnic Armenian forces backed by Armenia. Thousands of people were killed and hundreds of thousands were displaced.
Authorities in Nagorno-Karabakh declared independence that year, but it was not widely recognized internationally, leaving the ethnic Armenian administration there in a state of legal limbo and under blockade from Azerbaijan's government.
By 1994, when an internationally brokered ceasefire was agreed, ethnic Armenians controlled almost all of Nagorno-Karabakh, plus some surrounding Azeri districts that gave them a buffer zone and land bridge connecting their region to Armenia.
Azerbaijan vowed to take back control over the territory, using military force if necessary.
International efforts over the years to find a lasting peace settlement, involving France, the United States and Russia as mediators, have failed to clinch a deal.