Poland has had an interesting history over the centuries and it's borders have been constantly changing.
The largest extent of Poland's territory would have been in the 1600's, where it occupied territories known today as Ukraine, Belarus, Lithuania, Latvia and portions of Russia and Estonia.
But Poland has always been caught in the middle by opposing forces through history, such as the Teutons, Prussians, French, Austrians, Germans and the Russians (Imperial and Soviet).