evidence: Nagorno-Karabakh is part of Azerbaijan, but its population is majority Armenian. As the Soviet Union saw increasing tensions in its constituent republics in the 1980s, Nagorno-Karabakh voted to become part of Armenia - sparking a war that stopped with a ceasefire in 1994. Since then, Nagorno-Karabakh has remained part of Azerbaijan but is controlled by separatist ethnic Armenians backed by the Armenian government. Until recently, negotiations mediated by international powers had failed to deliver a peace agreement. [...] Modern-day Armenia and Azerbaijan became part of the Soviet Union when it formed in the 1920s. Nagorno-Karabakh was an ethnic-majority Armenian region, but the Soviets gave control over the area to Azerbaijani authorities.