We don't need to imagine it. Just look at the Cuban missile crisis, which can trace its roots by putting nuke-tipped missiles in Turkey and Italy, stone's throw from the USSR.
When the USSR parked its nukes 90mi away from the US, US committed an act of war by blockading Cuba, wordsmithed it to a quarantine; dropped signalling depth charge on a Russian submarine in international waters, which had nuclear-tipped torpedoes. The world's lucky that someone senior on that boat vetoed the launch.
I'm not saying one has to like Russia for its moves, but the west should at least look at itself in the mirror for once. Admitting Ukraine and Georgia into NATO amounts to blocking off the warm water port for Russia. That alone should explain why they would react against it.