There has been a lot of debate here about what exactly National Socialists mean when they say Polish crimes against the German minority. Unfortunately some forget important details or confuse events with others such as Danzig and Bromberg (No killings occurred in Danzig and Bromberg was after the war started) So I am here to give what information I have in the hopes people take note the next time these questions come up.
This book is essential to understanding the problem. I urge everything skip to the section on Poland and read, although if you have questions about Czechoslovakia or any other country that had diplomatic relations with Germany that bothered you than by all means read them as well.
https://ia801305.us.archive.org/13/items/HitlersRevolutionByRichardTedor_383/HitlersRevolutionByRichardTedor.pdf