By referring to original documents of the military and civilian institutions of the rebel Serbs as well as those of the Yugoslav People’s Army, this paper outlines the state and the activity of the aggressor forces in the occupied area of the Glina and Petrinja Districts, where Operation Whirlwind was carried out in December 1991. The military activities of the Croatian forces in this part of the Pokuplje region that preceded the operation are also described. A thorough insight into the preparation and course of the enemy counterattack in Operation Whirlwind is given, with a comparison of losses on both belligerent sides. Considering everything stated, the aggressor’s superiority in artillery and armour is obvious, as is the contribution of the Yugoslav People’s Army in carrying out the counterattack.