I recently learned that after WWI countries like Poland, Estonia, etc. fought against Russia and one another, apparently over borders. To my understanding, Poland invaded Russia, Czechoslovakia and Lithuania. But today, these "wars" seem to be forgotten, with maps usually only showing one post-WWI border: https://en.wikipedia.org/wiki/Aftermath_of_World_War_I#/media/File:Map_Europe_1923-en.svg
What exactly happened? Who wanted what, and did they get it? What was the mentality of attacking your neighbors so shortly after WWI? And why there is so little talk about these events today?
Ok, so I can't answer about the entire Eastern-Europe and there were certainly different aspects at play, but at the very core, for the new countries, it was a war of independence. When WW1 ended in late autumn 1918, German troops occupied a large portion of Eastern Europe, that they gained after defeating Russia in WW1, and now the germans were largely abandoning these occupations. What this caused was a power vacuum, where the retreating Germans no longer held control, the (slowly) advancing Russian factions (both communists, czarist loyalists and all other rebels) didn't yet hold control and there were many different local factions with their own interests in play.
Many new countries emerged from this chaos and they all had to fight for their independence (with the possible exception of Finland, that gained independence in 1917 and was already recognised by the Soviet Russia. But even this didn't spare them from war, as they were dragged to a civil war between the socialists/communists and the others.
But Estonia, Latvia, Lithuania and Poland fought for their independence (at first at least) and eventually managed to win or at least make a draw with the Soviets. Some new nations didn't manage to hold independence such as the Caucasian republics, which eventually fell under Soviet rule.
And Russia itself was in a long and very bloody civil war between many factions, there were a lot of shifting alliances, there were the British and Americans involved, among many many others.
And as these alliances shifted, so too did eventually the newly independent states fight between each other. The reason for that is largely the fact that ethnicities didn't just lie next to each other with obvious lines (borders) to separate them. Different people were mixed in these previously multi-ethnic empires (Russia, Austria-Hungary and even Germany). So it wasn't clear where to draw the borders and it led to some skirmishes.
I would say, that in the end, the wars fought between the new republics (such as Poland-Czechoslovakkia) were fairly minor and bloodless compared to the entire affair of Russian civil-war. But it still certainly poisoned relations in Eastern Europe and in the end might have proven to be the dividing reason, why Eastern Europe didn't manage to fight off the Soviet and Nazi encroachments in WWII.
