Myth about "Backstabed" poland
Information about Germany's proposals for Poland's territorial expansion to the east (USSR territory) in exchange for concessions regarding Danzig and the "corridor" is based on diplomatic documents, conversation notes, and the memoirs of participants in the events of 1938–1939.
Primary sources:
Conversation notes between Joachim von Ribbentrop and Józef Beck:
During a meeting in Berchtesgaden on January 5, 1939, Hitler hinted at the common interests of Germany and Poland regarding the USSR.
January 26, 1939 – Ribbentrop and Beck in Warsaw. The transcript of the conversation notes that "Mr. Beck did not hide the fact that Poland was laying claim to Soviet Ukraine and access to the Black Sea." Ribbentrop, in turn, discussed possible joint action against the USSR.
Dispatches of the French ambassador to Berlin, Robert Coulondre, who reported to his government in April 1939 that Germany was attempting to "mortgage" Polish foreign policy, promising political benefits in the East in exchange.

After poland asked for too much from germany and not receiving a deal poland got dissected as it deserves