[PzLdr]

Poland was invaded, in part, after they refused a sort of junior partnership in the Axis that would have required them to give Germany the Danzig Corridor, and Danzig itself, as well as free passage [and support] to the German Wehrmacht for an invasion of the Soviet Union.

Poland’s cooperation with Nazi Germany was limited to taking a piece of Czechoslavakia [along with others] when Hitler moved into the rest of the country in 1939.

[Lx]

You’re right. Poland got the corridor that separated the Reich from Prussia in the Versailles treaty. Germany actually had a legitimate beef on this one.

I forgot what biography of Hitler I read it in but in it, the Poles thought they could beat the Wehrmacht!

[LowTaxesEqualProsperity]

And Poland also took part of Lithuania, after issuing an ultimatum to them, before the war started, if I recall correctly.

[dfwgator]

Poland’s cooperation with Nazi Germany

There was no "cooperation" there, the Germans just chose to look the other way while Poland reclaimed land stolen from her in 1920s, it was a moot point to the Germans knowing what Poland's ultimate fate was to be.

[HenpeckedCon]

Poland was invaded because Hitler wanted “lebensraum” for the “arischen Herrenrasse.” Danzig was always just an excuse to invade. Hitler believed the Poles to be untermenschen barely above Jews so anything other than an abject surrender by the Poles would have been useless in preventing war.