Read this link:
https://www.wintersonnenwende.com/scriptorium/english/archives/articles/wrsynopsis.html
Before you say that the Danzig Massacre was just Nazi propaganda, please actually read the link, it hardly mentions such a massacre. These massacres did happen, even though the numbers are probably exaggerated. And no, the massacres against Germans in Poland was not 'in retaliation of the German invasion', because many of the bodies were discovered to be in advanced stages of decomposition, like pic related. They had been dead for a while, and their murders had been elaborate, brutal tortures.