Complex sentence: Shusha was the most important military stronghold that Azerbaijan held in Nagorno-Karabakh-its loss marked a turning point in the war, and led to a series of military victories by Armenian forces in the course of the conflict.
Split into simpler sentences: Shusha was the most important military stronghold that Azerbaijan held in Nagorno-Karabakh. Its loss marked a turning point in the war and led to a series of military victories by Armenian forces in the course of the conflict.