Here’s a timelapse animation of European borders from 400 BCE to the present: https://www.dailymotion.com/video/x1i1e6h. And just for context, modern Russia has over 190 ethnic groups.
It seems a bit specious to say that borders have all that much relevance to the U.S. supporting Ukraine at this moment in history.
The main reason we have an obligation to Ukraine is that we guaranteed them a security umbrella if, after the dissolution of the USSR, they would give up and destroy the nuclear weapons remaining on their territory. They agreed and the weapons were gone by 1996. Note that Ukraine was the heart of the Soviet aerospace sector. Nukes and aerospace are never a good mix. Here’s a short history from Brookings: www.brookings.edu/articles/the-trilateral-process-the-united-states-ukraine-russia-and-nuclear-weapons/
Lastly, during Vladimir Putin’s recent “interview” with Tucker Carlson, Putin mentioned Poland 30 times and capped off his remarks by accusing that country of forcing Nazi Germany to invade Poland. Are we going to be soon saying that we should not help defend Poland because its borders fluctuated and even periodically disappeared in the last 1000 years?
Just for bonus points, you can read through the Wikipedia articles on the First and Second Chechen Wars. These wars became the model for how Russia would conduct future wars in Syria, Mariupol, and now throughout Ukraine.
en.wikipedia.org/wiki/First_Chechen_War
en.wikipedia.org/wiki/Second_Chechen_War
Gene Dieken