If you want to imagine analogous situations, I think you should think back to when there was a Warsaw Pact and imagine how the United States would react were the Warsaw Pact suddenly to be in Prince Edward Island.
From a Russian point of view, this is strategic territory and always will be. When the Maidan trashed that EU-brokered agreement—those were the most fateful few hours in the history of modern Ukraine—and announced that there would be a downgrading of the Russian language and appointed well-known Russophobes to security posts and announced that it was going to join NATO, any sensate leader of the Kremlin would have reacted.
Now, you can argue that he overreacted—