After the successful Greater Poland uprising of 1806, it was regained by Poles and included within the short-lived Polish Duchy of Warsaw.

[6] On September 16, a detachment of Einsatzkommando 14 entered Kostrzyn, then gathered all the Polish men at the Market Square and carried out beatings and selection of 100 prisoners, who were eventually sent on foot to a newly established camp in Swarzędz.

[8] On October 20, 1939, the Germans carried out a public execution of 28 Poles, including teachers, school principals, merchants, craftsmen, etc.

[9] It was one of many massacres of Poles committed by Germany on October 20–23 across the region in attempt to pacify and terrorize the Polish population.

[13] Most of the Germans responsible for the crimes against Poles in Kostrzyn fled the town before the approaching Eastern Front and escaped justice.