I haven't read the book, but I can only imagine that a lot of creative liberties would need to have been taken - otherwise a surprising number of these countries wouldn't have existed at the relevant point in history.

eg, Poland was part of Russia during the 1850s Crimean War, and a little poetic license could stretch that a long way.

Having found further references, it seems most the book's claims for Poland hinge on actions in & around Danzig while it was part of Prussia - now being Gdansk, part of Poland.