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.
Another national security issue is the Russian warm water port in Crimea. This was traditionally Russian territory until Khrushchev transferred it to Ukraine in 1954. Putin has already taken Crimea back for Russia, but it has no overland connection to Russia. Contact with Russia must be over Ukrainian territory. Putin may not find this acceptable, but so far it sounds as if there may be room for negotiation. If the pro-Russian, eastern provinces of Ukraine were granted lots of autonomy by Ukraine, so that Putin felt he could rely on this for transport to and from Crimea, he might not feel that he has to annex them as part of Russia. It remains to be seen what assurances Ukraine will give and whether Putin will accept them. If not, he may feel that he has to take eastern Ukraine militarily.
Discussing strategic access by Russia to Crimea avoids the issue of whether Russian has a special obligation to Russian speaking, Russia loving populations in surrounding countries. This is the issue that brings fear to the Baltic republics. They might prefer to see the Ukrainian issue resolved without getting into the question of what to do about ethnic Russians in countries bordering Russia.