To what extent does Putin see the crisis in Ukraine the same way that Kennedy saw the Cuban missile crisis: a foreign military threat to the national security of the country? It is not clear what NATO is going to do vis-à-vis Ukraine. Ukraine is not a NATO member; so, NATO has no treaty obligation to defend it, although it does have treaty obligations to Poland and the Baltic states. Who knows what Putin thought, but it would be reasonable to see Ukraine (and Belarus) as a buffer between Russia and the NATO allies, a kind of a Finland, as many commentators have described it. He counted on his puppet rulers in Ukraine to keep the lid on yearnings to join the West, but they failed him while he was busy with the Olympics. While there is a lot of talk about Ukraine never joining NATO, who knows what might happen in ten or twenty years.
On the other hand, it is arguable that NATO is not a threat to Russia,as long as Russia behaves itself and does not engage in aggression. In the past there was some talk that Russia itself might join NATO.
This may be where the sense of Russian greatness comes in. Russia has always been on the border of Europe, not quite European, but always interacting closely with Europe, whether under attack by Napoleon or Hitler, or engaged in a cold war, or in a trade dispute with the EU. Russia has historical justification for distrust of Europe. Now Russia’s first capital city, Kiev, is looking to the West to join the EU rather than to the East as an ally of Russia.
Despite the historical and military consequences for Russia, does Russia have any right to interfere in the self-determination of the Ukraining people? If the US experience with the Cuban missile crisis is relevant, them the answer might be yes, if there are legitimate national security risks for Russia. The West says, no, there is no national security risk, because NATO and the West will never be an aggressor against Russia. For Russia, the question is whether that assurance is one on which it can stake its existence for the foreseeable future.