[PAR35]

Well, the free city of Danzig and the ‘Polish Corridor’ had been carved out of Germany after World War I.

[dfwgator]

In the 1920s, the Poles knew they most likely wouldn’t be able to keep Danzig, so they built their own port in Gdynia.

[Cronos]

It's not that simple - Danzig was heavily German, but there was a sizable Polish minority and the hinterland (what later became the corridor) was Polish (well Kashubian) majority.

"Germany" in 1914 had carved up a number of neighbors, so when it (and Hungary) were carved up after losing, that's when the lands went to other states.

However all of that is moot - Hitler wanted to eliminate Poland as a whole - and he did that between 1939 to 1945 - Gdansk/Danzig was an excuse.