The political situation in the Socialist Federal Republic of Yugoslavia (SFRJ) and Croatia aggravated in 1989 and during 1990 and 1991. The position of the Croats in the SFRJ was unfavourable, and other peoples, especially Serbs, were also dissatisfied with their position. The whole situation in the SFRJ reflected on the Slavonia area and its northern part, namely the area of Bilogora. In Grubišno Polje, the political situation since the end of 1989 was more tense. In the second half of 1991 armed attacks by Serbian rebels in the Bilogora area began. The military activities of Croatian forces became necessary. Military-police operation Otkos-10 was the first successful and well prepared action of the Croatian forces, id est the forces of the Croatian National Guard, the Croatian Police and the National Defense Detachment. The operation took place in two phases: the first one lasted from October 31st to November 4th 1991 and the second one from 11th to 12th November 1991. The operation involved 2847 Croatian defenders, who dismissed the forces of the rebel Serbs from Bilogora in the east direction to Papuk. The majority of the Serb population in Bilogora withdrew with hostile forces. After the Otkos-10 operation, the operations Orkan-91 and Papuk-91 followed in the area of western Slavonia. The forces of the rebel Serbs and Yugoslav National Army (JNA) were repressed towards the southern part of the West Slavonian battlefield. The operations were interrupted by the entry into force of the Sarajevo Truce on 3th January 1992. After the implementation of those operations there were still about 699 km² of territory of Western Slavonia under occupation. The operation was followed by the arrival of UN peacekeepers in 1992. The rest of the occupied area was finally released by the Bljesak operation in 1995.