The Russian Black Sea fleet is based at Sevastopol (as it has been for eons) under a 20-year lease that expires on 2017. The Ukrainian government has made it clear that Russia can forget about renewing the lease.
But there is another dimension to this — the balance of power. Russia knows that its invasion of a democratic U.S. ally and forcible alteration of its borders is a heavy strategic blow to the United States: it makes a mockery of the value of an alliance with us. Russia must be made to see that its action will be answered by an even more grievous strategic blow. The loss of Sevastopol — Russia's equivalent of Norfolk — is the perfect punishment. And what a great new base it would make for the U.S. Sixth Fleet.