That’s very recent domestic affairs of the past century you are referring to. Suggest you go back a few more centuries or a couple of millenniums. There were many shenanigans and domestic violence in between but they are the closest of Slav tribes. Russians are part Slavs but a mix of Sami, Roma, Finn,Viking & even Mongolian as you can see on Putin’s face. Ukrainians got good reasons to go with Poles in time of need.
