The meaning of the word Poland has changed during the centuries, according to wars and treaties.
The Two Nation Commonwealth (Rzeczpospolita Obojga Narodów), from 1569 to 1795, corresponds to the maximum expanse of Poland. It was built from the union of Poland and Lithuania and ended when Poland was partitioned between Russia, Prussia and Austria.
Poland’s resurrection in 1918 yields a larger extension. In 1945, its borders are moved to the west and the German territories of Dantzig and of the western part of East Prussia became Polish?
Therefore, a part of the territories at the east of Poland, and presently in Lithuania, Belarus, Ukraine, belong to Polish history. Conversely, part of its history is found in Moscow, Berlin and Vienna, capital cities of the empires which formerly governed Poland. A genealogist must take this into account. He might need to look for his Ukrainian ancestors in Warsaw or in Vienna for those who lived in Cracow.
Note : the term “Great Poland” (wielkopolska) refers to one of the historical provinces and voivodeships of Poland.