Answer
Ethnic conflict in Nagorno-Karabakh was the main cause of the First Nagorno-Karabakh War that Ter-Petrosyan had to lead Armenia through. The conflict began in 1988 when Armenians in Nagorno-Karabakh declared their independence from Azerbaijan. Fighting broke out between Armenian and Azerbaijani forces, and by 1992, Armenian forces had captured most of Nagorno-Karabakh and surrounding areas. The war ended in 1994 with a ceasefire agreement, but the conflict remains unresolved.