I'm very curious about life in Poland prior to World War II. My understanding is that after WWII, a series of population exchanges essentially made Poland an ethnically homogeneous state. However, the Second Polish Republic had different borders that incorporated significantly more non-Poles, particularly in the Krezy region. Were these ethnic minorities allowed to express their national identities, or were they pressured to "Polanize", for lack of a better word?
I can speak only about Ukrainian territories. Polish government tried to gradually assimilate local population, first of all by using only Polish language in all public education and converting Orthodox churches to Catholic ones. Moderate Ukrainian parties and movements were legal, but Nationalists and Communists could exist only underground and sometimes used violent methods. In retribution, government increased repression and incited pogroms against Ukrainians in the early 1930s, after which came "normalization" of relations, and then again increasingly hostile treatment. Unlike in Soviet Union, however, Ukrainians were able to create a vivid civil society outside of state-controlled areas of life - private schools, cultural clubs, cooperatives etc. So in general attempts at Polonization of these lands failed.
I learned this from a book by Yaroslav Hrytsak, Essays in Ukrainian History: Making of Modern Ukrainian Nation.