The Katowice massacre or the Bloody Monday in Katowice that took place on 4 September 1939 was one of the largest war crimes of the Wehrmacht during its invasion of Poland. On that day German Wehrmacht soldiers aided by the "Freikorps" militia executed about 80 of the Polish defenders of the city. Those defenders were self-defense militia volunteers, including former Silesian Insurgents, Polish Boy and Girl Scouts, and possibly a number of Polish soldier stragglers from retreating Polish regular forces who joined the militia.