A talk by Professor Mearsheimer 6 years ago explained very well the motive of Putin. https://youtu.be/JrMiSQAGOS4A
First, the point usually missed by everyone is that NATO is a nuclear power and it is hard for Russia to swallow a nuclear power adjacent to it. So Russia reacted so violently as Ukraine tried to join the NATO.
Second, Ukraine is not Poland. It actually was the origin of the Russian culture. There is a deep psychological attachment of the Russian people towards Ukraine.
Third, eastern Ukraine has a majority of Russian population. In particular, Crimea has over 60% Russian majority even before its annexation by Russia. And according to wikipedia, Putin has high support in Crimea, securing 92% of vote there vs 77% over the entire country in the 2018 presidential election. Moreover, a 2019 survey indicates a majority of 54% of Crimean support Russian governance. But the west simply ignores this and still considers Crimea a part of Ukraine.
More than 6 years ago, Professor Mearsheimer’s recipe for peace is to keep Ukraine a buffer state. That is exactly what Putin asked for. Unfortunately, it is too late now. The modern world will not and should not yield a request by force. Not to mention that Russia is losing now. But what is scary is that we are essentially pushing Putin to go nuclear.
The ultimate cause of this doomsday crisis is nationalism, both on Russia and Ukraine sides, and with the encouragement from the west to the latter. When we empathize with the Ukrainians to seek independence from Russia, we should accept the will of Crimeans and the residents of Eastern Ukraine to seek independence from Ukraine as well. If we want to avoid extinction as a species, all of us should abandon nationalism and our pride towards our race, nation, and country. If Hong Kong and Tibet want to separate from China, let them be. And so do Scotland from the UK, Quebec from Canada, and maybe California from the US. Provided that it is really what the locals want.