Not sure that's relevant in this instance, Euro. Russia took control of Crimea back in the last war, and the Red Navy have always had other ports on the Black Sea (even after Ukraine became independent).
The reasons for this incursion are three-fold, imo.
Firstly, he wants to arrest any further NATO expansion on Russia's western flank, and Ukraine and Belarus are the only remaining bulwarks. If Ukraine were to join NATO, that would leave Belarus, and he probably knows that the Moscow-supporting autocracy there have been living on borrowed-time for a while now.
Secondly, he is a Cold War KGB relic, who has long thought the dismemberment of the USSR was a historical travesty.
And thirdly, he is a maniacal, bas*tard gone mad with power - as all dictators eventually do.
The thing I don't get is that he controls more wealth than anyone else on the planet. Why would he risk everything he has gained (putting aside the nefarious means by which he achieved that control), for something which is going to compromise that position? One can only assume that the fever has overtaken him, and he is no longer thinking rationally.....which is very worrying indeed.