Nobody said anything about threatening them with nuclear weapons, just conventional ones. Countries are not permitted to attack and invade other sovereign countries either and yet here we are with Russia doing just that. So why should we care about technicalities concerning the Black Sea?
They will not be deterred except by a show of and a threat of force. The world is making the same mistake in thinking that an expansionist megalomaniac can be satisfied with only a little bit of conquest.
The US is on thin ground when it comes to treaty commitments remember that we promised Serbia at the end of the Kosovo War that Kosovo would never be given independence and would remain as an autonomous part of Serbian. A few years later, we were demanding a promoting independence for Kosovo.
That was a mistake, I agree.
Then there also promise that if Germany were united, the US would agree not to expand NATO "even one inch" into former Warsaw Pact countries.
Putin trying to reconquer former USSR colonies shows clearly that NATO should have been even more receptive toward allowing them into the NATO fold. Not that this is merely Obama's failing. Bush didn't do anything when Russian tanks rolled into Georgia either.