Greiser typified the brutality exhibited by officials of Nazi Germany to the Poles. He was an ardent racist who enthusiastically persued an 'ethnic cleansing' program to rid the Warthegau of Poles and to resettle the 'cleansed' areas with ethnic Germans. This was along the lines of the racial theories espoused by SS Reichsführer Heinrich Himmler. Mass expulsions of Poles from the Warthegau to the General Government and summary executions were the norm.
After the war, the Poles tried him for war crimes. He plea that he was only following orders did not hold up as it was shown that other Gauleiters did not follow a similar policy - for example Albert Forster, Gauleiter of Danzig_West Prussia (the other German_annexed section of occupied Poland) simply declared all Poles in his area of responsibility proficient in the German language to be Germans. After Greiser was convicted, he was paraded around the city of Poznan in a cage, before being hanged.