Such a belligerent neighbor that they had similar levels of aggression as the rest of Eastern and Central Europe.

What you're looking at is the inherent flaw of Progressive Nationalism. The claim that a government has an inherent obligation to represent all of an ethnic group, wherever that group may lie. If they are a majority of a region, then that land should fall under the control of a government. If they are a minority, then the government should step in to protect them. They may even offer duel citizenship to the ethnos within that political boundary, then claim that "discrimination" gives them the right to invade across that political boundary and claim it for themselves. Germany did all the same shit. As you noticed, sometimes in the same place at the same time.

Poland's rush for land and power after having come into existence is what it is, but it's also not too different from what happens when any other countries emerge from the collapse of empires, and it also not an excuse to start making ridiculous claims that Germany was the international dindu nuffin.

If you really want to argue that Progressive Nationalism is the problem, then fine, but blame Progressive Era Leftism and Woodrow Wilson.