I think the reasons for Russian invasion is somewhat simpler: - Ukraine military equipement is aging and of Russian origin. It is mostly useless to defend ones

I think the reasons for Russian invasion is somewhat simpler: - Ukraine military equipement is aging and of Russian origin. It is mostly useless to defend oneself against Russia. One day or another an independant Ukraine would have bought military equiment to the west or China. This is inacceptable to Russia for the next reason. - traditionnel defense paradigm of Russia is "defense in depth". Refer to Napoléon Wars or WW2 for applications of this paradigm. Since Putin seems to stick to it, Ukraine is merely a buffer for Russia, so long it is an ally. Hence the urge to replace the Ukrenian government with dummies as in Belarus. - Allies can only have compatible weapon systems.