They simply invaded their country and grabbed a portion of their land.

You’re ignoring the histories of these areas. I’m not saying they’re angels or anything but ethnically Russian people on their own lands have every right to not want to be ruled by Ukrainians (or the EU), especially in the regions where they’re a large majority. They were only separated from Russia 30 years ago, based on a treaty drawn up by corrupt leaders. Any nationality would react in the same way.

Germany was in a similar situation, but arguably much worse following WW1. When you humiliate a nation, chop up their borders & territory ignoring ethnic realities on the ground, it’s going to cause problems down the line.