The predominantly Armenian-populated region of Nagorno-Karabakh was claimed by both the Azerbaijan Democratic Republic and the First Republic of Armenia when both countries became independent in 1918 after the fall of the Russian Empire, and a brief war over the region broke out in 1920. The dispute was largely shelved after the Soviet Union established control over the area, and created the Nagorno-Karabakh Autonomous Oblast (NKAO) within the Azerbaijan SSR in 1923. During the fall of the Soviet Union, the region re-emerged as a source of dispute between Armenia and Azerbaijan. In 1991, a referendum held in the NKAO and the neighbouring Shahumyan Province resulted in a declaration of independence. Ethnic conflict led to the 1991–1994 Nagorno-Karabakh War. Conflict has sporadically broken out since then, most significantly in the 2020 Nagorno-Karabakh war.