"Had to take on both the nazis and the Soviet union" is reactionary polish revisionism.
The second "Republic" of Poland, after collaborating with nazi Germany to carve up Czech and Slovakian lands, was assured that there would be future collaboration with the nazis against the asiatic Red hordes in the east. The fascists grabbed them by the ear and started kicking them in the ass so bad through a surprise invasion the legal polish government fled out of the country so fast it was impossible to get a hold of them to communicate any orders to the Polish State.
When the Soviets stepped into "Polish" lands, it was a lawless land with no government authority. With little resistance from the people, the Soviet Union reclaimed Western Ukraine and Western Byelorus that had been torn from the Ukrainian and Byelorussian Socialist Republics by the German Kaiserreich in the Treaty of Brest-Litovsk.
Asserting that the Soviet Union invaded Poland is a cold war lie on behalf of American imperialism.
God dammit, I believed that my entire life. What books or articles do you recommend to learn more about this?
Depends, this, on the section of '38 gives some cursory info on the land-grabs And this details the Soviet-Polish war of 1920 in which the fledgling Soviet republics sought to liberate lands stolen by the Germans and granted to the reactionary nationalists during ww1's Treaty of Brest-Litovsk.
They're good rough starts on the subjects to get an idea of the bigger picture of the region.
