One Russia
If I understand President Putin’s argument for all this invading he is doing it is to reconstitute the true Russia. We in the west call that the Soviet Union plus the Warsaw Pact countries, but Putin may have different notions of what “Russia” ought to look like. The problem is that those other countries like Ukraine, no longer want to be part of Russia, if they ever did. They see themselves as independent, sovereign entities with no wish at all to be realigned with Putin’s Russia. In other words, Putin looks at all of this land mass, that in the current Russia and that in what used to be the Soviet Union, as one Russia. He views this one Russia as the bulwark against western aggression, but these independent countries want to be part of NATO and the European Union, and more importantly the populations of the countries want that, too. They want freedom, autonomy and self-determination. Putin wants them to heel to his desires and cow to his weapons. The Ukrainians have basically said no to that and look like they will fight to the death to prevent there from becoming one greater Russia again.