During the German invasion of Poland, which started World War II, Nazi Germany carried out a number of atrocities involving Polish prisoners of war (POWs). During that period, the Wehrmacht is estimated to have mass-murdered at least 3,000 Polish POWs, with the largest atrocities being the Ciepielów massacre of 8 September 1939 (~300 victims) and the Zambrów massacre of 13–14 September (~200 victims). Most of those atrocities are classified as war crimes of the Wehrmacht. Jewish soldiers with the Polish Army were also more likely than others to be victims of various atrocities.
A number of other atrocities against Polish POWs occurred later in the war, particularly on the Eastern Front, with the largest atrocities in 1945 committed at Podgaje (~200 victims) and Horka (~300 victims).