Access to the Black Sea (Their Fleet at Sevastopol) and access to the Atlantic though the Bosporus is far more important. Turkey controls those and they are in NATO. Invading Russia through Ukraine would have little strategic merit.I'm not for a second defending Russia or suggesting they have a right to invade Ukraine here or anything but it is worth considering both sides of the dispute. Fear works both ways. The Ukraine border with Russia almost doubles the combined total of the NATO countries who border it.