Complex sentence: According to Bogusława Marcinkowska, a historian from Kraków's office of the Institute of National Remembrance, the Ukrainians threw infants against walls and cut open the stomachs of pregnant women.
Split into simpler sentences: According to Bogusława Marcinkowska, a historian from Kraków's office of the Institute of National Remembrance, the Ukrainians exercised sadism and cruelty. They threw infants against walls and cut open stomachs of pregnant women.