Surely what was to deemed to be Polish territory depends on how far back in history one goes as its borders ebbed and flowed through its own conquests and alliances just as it was conquered and divided by other alliances or empires throughout the centuries. One could argue that even today Poland does not exist if one looks at the 1939 map of Europe—it is clearly not where it once was last seen by my father! Poland definitely did not exist between 1939 and 1945 if one looks at German and even some US published atlases of the time. This is clearly why the Nazi extermination camps were never built in Poland although they are situated in the Poland of today.
For example a snapshot map of Poland March 1919 before Poland seized the territory of the West Ukranian Peoples Republic in July 1919. Then there was the so-called Polish-Lithuanian War lasting from August 1920 to October 1920 an armed conflict over control of the cities of Vilnius (Wilno), Suwałki and Augustów... and many more. Hopefully the Poland of today will remain as she is for evermore!
Wiki link to Poland's territorial history with maps