The boundary became an international frontier in 1991 following the dissolution of the Soviet Union and the declaration of independence by Armenia, Azerbaijan and Nagorno-Karabakh in 1991. Azerbaijan abolished the NKAO, sparking a full-scale war with Armenia. The war ended in a ceasefire in 1994. This left Armenia in control of the vast majority of Nagorno-Karabakh, organised as the Republic of Artsakh, and much of Azerbaijan proper, including the strategically vital Lachin Corridor. Since then the conflict has remained frozen, creating the modern de facto border between the two countries which follows the de jure Soviet-era border only in its northern half. Since the ceasefire relations between the two countries remain extremely tense and there have been numerous flare-ups of fighting along the border, notably in 2008, 2010, 2012, 2014, 2016, 2018 and 2020.