On September 3, 1939, two days after the beginning of the German invasion of Poland, highly controversial killings occurred in and around the Polish city of Bydgoszcz, with a sizable German minority, located in the Pomeranian Voivodeship.
According to the traditional Polish version, the incident is related to the activities of groups of German saboteurs attacking Polish troops behind the front lines. As a contingent of the Polish Army from Pomerania was withdrawing through Bydgoszcz it was attacked by German irregulars from within the city and reported to be engaging enemy snipers. In the ensuing fight both sides suffered some casualties; captured German nonuniformed armed insurgents were executed on spot and some mob lynching was also reported.
The killings were followed by German reprisals and oppression, including a "de-Polonisation" campaign. In an act of retaliation, hundreds of Polish civilians were picked at random and executed by German military, including by units of Einsatzgruppen, Waffen SS and Wehrmacht, with further reprisals soon to follow. After attacks by Polish snipers on German troops in Bydgoszcz continued for several days, German governor, General Walter Braemer, ordered the execution of 80 Polish hostages over the next few days. By September 8, 1939, between 200 to 400 Polish civilians had been killed.
more at Wikipedia