Assumptions
Russia's nuclear weapons sufficiently deter its opponents.
Russia suffers from a persecution complex.
Evidence for Russia's fear
Russia itself has hankered after buffer zones and states like Belarus and Ukraine.
The European Plain makes Russia vulnerable and indefensible, as in past invasions of Russia.
From Ukraine, an enemy can cut off Russia's access to the Black Sea and Southern Russia.
My questions
But who's itching to invade Russia? Who can even invade Russia sufficiently? Who would risk mutual assured destruction, a nuclear apocalpyse, and human extinction?
If the answer to question 6 above is no, then why does Russia still suffer from a persecution complex?