Is the seemingly ever-lasting divide following the former German Empire's borders due to German influence and infrastructure, or to the Polish people being resettled there from the east after 1945?
Neither and both at the same time. :-D
Long answer: It's about structural differences. Former Congress Poland's and Galicia's agriculture was mainly based on small peasantry (although there was of course also a large number of small nobility), and unlike in other Eastern Bloc states that was not too much affected by collectivization during communism. The Eastern parts of the German Empire (i.e. today's Western Poland) on the other hand had a lot of large estates with the bulk of the population being farm laborers rather than small peasants. It was also much more economically developed, I remember someone on the forum posting maps of the railroad network in 1914, the contrast was quite impressive.
Resettlement after 1945 only reinforced this, but it cannot have been the main factor, because why would Greater Poland (i.e. the region around Poznań) be a "liberal" stronghold otherwise? I mean, it was majority Polish even before 1918 and most of it became part of Poland after WWI.
