The German Invasion of Poland was the catalyst for the Second World War, in much the way that the assassination of Archduke Franz Ferdinand was the catalyst for the First World War. WWII was not fought because Poland was invaded, just as WWI was not fought because the Archduke was shot.

It was what the Invasion of Poland represented-renewed German aggression aiming to enforce German hegemony over the continent-that lead to the Second World War beginning when it did. Hitler had demonstrated with his occupation of the Czechlands that year that his word was worthless, and that he was bent on conquest; Poland served essentially as a trip wire, a line that were Hitler to cross it, would require Britain and France to step in. The Allied victory in the Second World War checked German expansionism once and for all, and did the same to Germany's allies Japan and Italy.

It was tragic that, having given so much for the Allied cause, Poland traded one cruel master for another. However, it's worth pointing out that if Hitler had had his way, the largest concentration of Polish-speakers in the world today might be Eastern Philly :(