There was never any question of Russia losing its Black Seas base (at least, not until now). The base is on long-term lease and at no time has the goverment of Ukraine threatened to cancel it. This notion is a non-starter.
Nor is there, Russian propaganda aside, much real appetite on the part of Ukraine’s ethinic Russian minorities to actually rejoin Russia, Crimea aside.
This has nothing whatsoever to do with Russia protecting its “legitimate interests” or “ethnic minorities”, and everything to do with Putin’s rage at having his puppet ignominously kicked out, and fear of losing his new-style Russian imperium, and overturning the possibly infectious “bad example” of people successfully defying authoritarianism.
Similarly, the talk of the West going to war is all nonsense. It will not happen.
What will likely happen, is an economic disaster for Russia. The economy of Russia was not doing well, even before this move. That 50 Billion dollars Russia spent on bolstering its international image with the Olympics has all now been wasted, the Russian stock-market and currency are in free-fall, investors are scared off in droves, and that’s before any sanctions have been imposed or any real costs of military adventurism have been paid.
Naturally, it will also be a disaster for Ukraine, and to a lesser extent, to everyone else. But there is no reasonable way Russia can afford much in the way of military adventures.