Russia saw the further encroachment of the Black Sea area by Nato as a threat to Russia's national security (just as the US saw the USSR encroachment on the USA by putting missiles in Cuba in 1962).....(this is not rocket science...eh?)
So far Russia has been successful (at enormous cost) of achieving that objective because the Ukraine, Georgia or Moldova have not joined NATO (yet) because according to NATO's membership they do not qualify because of disputed borders (engineered by Russia)
This behaviour by Russia is all about it exercising its power militarily militarily when diplomacy failed due to NATO's (USA) beligerancy in order to firewall Russia's sphere of influence to the west and to induce an economic collapse of Russia (as it did the USSR in 1991) so that Russia would be incapable of maintaining its Nuke arsenal and the means of delivering it.
Since 1991 the USA has been the only global power and it simply want to keep it that way .
Prior to 2014 Russia was developing its Ultrasonic Nuke delivery systems and upgrades of its Nuke arsenal and subsequent western sanctions were designed to economically make a "collapsed USSR" out of the Russian Federation.
The "NATOfication"of the Ukraine was never going to work because it would have allowed NATO (primarily US WMDs) to be located on Russia's Black Sea and Sea of Azov borders thereby rendering the Russian Black Sea naval fleet potentially dead ducks and the eventual expulsion of Russia from the Crimea when the current lease was due to expire in 2042.
