The Soviet offensive caught much of the eastern Poland virtually undefended, as most of the Polish forces from the area had already been transferred to the German front. After breaking through overstretched defenses of the Border Defence Corps, the Soviet 15th Armored Corps started a fast advance towards the city of Grodno. Commander of the prewar Grodno Military Area Command, Gen. Józef Olszyna-Wilczyński together with the mayor of Grodno Roman Sawicki began organizing city defenses, basing mostly on march battalions, volunteers, Boy Scouts and police forces.