of thousands of political prisoners in the initial stages of   Operation Barbarossa . [153] [154] September 29, 1941 Ukraine Babi Yar massacre 30,000 [155] Nazi   Einsatzgruppen   killed the   Jewish population of   Kiev . [155] [156] [157] [158] [159] October 20–21, 1941 Serbia Kragujevac massacre 2,796-5,000 Nazi