It is looking like a bit of a war is shaping up in the Ukraine. The new government has issued call-up papers to men of military age and the Russians are building up their forces on their side of the straits of Kerch which separates Russia from the Crimean peninsular. There's about a one mile water gap between Russia and the Ukraine.
But why the Russian interest in the Crimea? Well, because it gives access to the Mediterranean. The deep water port at Sevastopol is the home of the Russian Mediterranean fleet. Time was they also used Odessa but they have pretty much been thrown out of there now. So if Russia wants to have any influence in the Middle East or North Africa they need somewhere to park ships in the Black Sea.
Without Black Sea ports they are reduced to operating out of the Baltic and ships out of the Baltic tend not to reach the Med if the Americans don't want them to. (A couple of years back the Russians tried to supply the regime in Syria out of the Baltic but that ship got turned back. It needed to refuel somewhere to make the journey and was denied everywhere after the Americans put a hex on it. That's just plain embarrassing for a country as big as Russia.)
The Russians do have the entire Sea of Azov and then some Black Sea coast line on the Eastern shores (which they annexed from the Ottoman Empire as it was collapsing 150 years ago.) But the water is darned shallow and getting shallower all the time.
The Black Sea is emptying into the Med. It's still deep in the West but gets shallower as you move East. The Sea of Azov is knee high in parts. You wouldn't get an aircraft carrier in there although you might be able to cross it with a tank.
So this is why the Russians have coveted the Crimea for centuries. There is natural deep water. This is why the Soviet Union moved so many ethnic Russians into the area and the Tzars before did the same.
The Russians do have another option. There's a port city on the Eastern shore called Novorossijsk but they have only recently started putting investment there. It will be decades before it is up to spec, and all the while the water is draining away. So at the moment all Russian eggs are in the Sevastopol basket.
After last week's American-sponsored coup-d'etat in the Ukraine it looked like the Russkies were going to be pushed right out of the Med. It's not surprising they are all over this. The "protecting ethnic Russians" is as good an excuse as any.