Complex sentence: The JNA's planned campaign included an advance by the 9th (Knin) Corps in the Šibenik area, who were tasked with isolating Dalmatia from the rest of Croatia.
Split into simpler sentences: The JNA's planned campaign included an advance by the 9th (Knin) Corps in the Šibenik area. The corps was assigned the task of isolating Dalmatia from the rest of Croatia.