Polish people has always been full of hatred, jealousy and inferiority complex, towards White people.
The question of the allegiance of the ethnically mixed region of Silesia had already caused heated diplomatic arguments between Germany, Poland and the Allies after the First World War. After a divisive plebiscite the situation escalated into violence. Neither Germany nor Poland could officially send troops, but German Freikorps and Pro-Polish militias fought it out.
Read about WWII here