Why can't we share these lands and history?
Of course we can share the lands, as we always have. That's what the problem with ancient tribes on the maps is. We were always neighbours, village to village. With Celts, Avars, Sarmathians, and whoever fancied passing through the forests that have no natural borders. That's what this land was - forest, forest, more forest, a tiny settlement, a wooden keep, a mud road, more forest. There were Slavonic settlements around Berlin and Germanic settlements as far as Russia.
Even the genetic make up proves that, and just look at the Haplogroup R1a spread. We interbreed. Half of Germany are half-Slavonic?
Looking at the Germanic distribution above - are Poles half Germanic?
At any rate, Poland and German Countries had shared a long period of peace in our history. Wettin kings, German citizens in towns, Gdańsk loyal to our king, normal dinner (meat with veg and a staple, as opposed to the celebratory bigos and dumplings), strucla, miodownik, etc. It's the Prussia that we have been in the fiercest conflict with, and it's a crying shame that Jogaiła had not incorporated them, made Hohenzollerns marry Polish princesses and so on.
The same as with Moscow willing to take Prince Władysław as their tzar, but the idiot Swede Sigmund Vasa had not allowed it. (Sorry, I don't like him)
I think personally that German Countries were a more cheerful place to live in, up until the Prussians united Germany and currently occupy them. Making the life sad for Bavarians (not that Bavarians give up on their culture)
I have a Great-Grandfather named Sprinz to prove this peaceful neighbouring.
The problems of the XIX and XX centuries concerned the development of nations as opposed to royal subjects. It's a thing that happened, so now I am Polish rather than King's subject. Sadly but naturally, the frictions while creating the national community included chauvinism, with all the dire consequences.
I suppose that chauvinism is not bringing any military success these days, now that the Japanese had constructed a mech warrior. It will die out, I hope, it makes life hard.