It didn't help that in Poland was an aggresive country in 1930s too, and was waging aggressive wars against its neighbours (read, "Lithuania"), and occupying and bruttaly polonizing what was never its ever. Not too say that Lithuania was not run by own nationalists, who also managed to get into military conflicts with Latvian "brothers". The moral is that the whole Europe was boiling in every own nationalism that culminated with WW2. Thinking that it can never repeat is as delusional as 100 years ago
Nationalism truly was the mindkiller that enabled WW1 and WW2. It's important to remember that although WW2 is rightfully remembered for the Holocaust, it was in essence an attempt at a European colonialism by Germany coupled with a Germanic ethno-nationalism (or more accurately: völkisch nationalism, which is still deeply entrenched today).
The racist pseudoscience of the Germanic übermensch served the same purpose as the skull measuring scientific racism used to justify the enslavement and extermination of the native population in African colonies. "Lebensraum" was just a Germanized version of Manifest Destiny.
The colonization of Africa, India and the Americas involved massive displacement of natives and mass killings that at least colloquially can best be described as genocides. Much the same way as forests were cleared to create farmland, native populations were cleared to create "living space" for settlers. Much like natural resources were exploited and brought back home, natives were used for manual labor until they were used up.
Nationalism allowed leaders to use a national identity to rally their subjects against their neighbors. In an act of massive hubris and severe underestimation of technological advancements this nationalist fervor led to the mass death event that was WW1. WW2 in turn built on this but also brought colonialism home with the explicit goal of not only redrawing borders but also repurposing the land and eliminating any natives standing in the way.
I.e. Poland may have engaged in nationalist assimilation in the form of Polonizing Lithuania but Germany explicitly wanted to remove anyone not part of the "volk", at the barrel of a gun if necessary.
