Causes
Field Marshal Wilhelm Keitel had issued an order on 16 September 1941, applicable to all of occupied Europe, to kill 50 communists for every wounded German soldier and 100 for each German soldier killed. German soldiers were attacked in early October by the Communist Partisans and by Chetniks under Draza Mihajlovic near Gornji Milanovac, and the massacre was a direct reprisal for the German losses in that battle.
A German report stated that: "The executions in Kragujevac occurred although there had been no attacks on members of the Wehrmacht in this city, for the reason that not enough hostages could be found elsewhere."
Read more about this topic: Kragujevac Massacre