What destabilised the Ukraine at the start was that their prime minister signed some treaty that brought them closer to Russia, whilst the south / Crimea was happy with that the north wasnt.
There have been protests in Kiev for almost half a year now. I think it was in January or February when 20 protestors were shot by government snipers? Shortly after the old prime minister left. Except now that he's been replaced it's not entierly simple, the north of the country like the new guy. The people (protestors) in the Crimea, don't see the guy as legitimate.
The trouble now is that the Crimea is only a part if the ukrain due to an administrative change some 60 years ago. If a bounded hadn't been changed for who looked after what people the (mostly quite rich Russia descended) people of the Crimea would still be Russian not Ukrainian). -in many ways think if it a bit like Pakistan/India, Israel/Palestine or most of Central Africa etc, basically someone took a map and a pen and said that bits mine and that bits yours, with no real thought for the history, culture or feelings of the people who actually lived on that land!
The biggest problem however is as follows. Some 30 years ago the Ukraine had the third largest stockpile of nuclear weapons in the world (Russia, America Ukraine I think)
Or maybe Russia China Ukraine then the us. The tiny country had more nukes than most of Europe put together.
AND they gave them up, signing a treaty with Russia, uk/France/European nuclear powers, and the US,
Russia can't simply send troups in, and if they do, we. (As in you Americans and us British) must
go to war.
The consequence of not going to war is to make all idea of non poliferation of nuclear arms a joke, it shits all over any kind of diplomacy to stop North Korea or Iran from developing nuclear weapons because it shows that even though we say that we (as nuclear powers) will protect any country that agrees to give up nuclear weapons when it comes to it we sort of shrug and look the other way! If we don't actually do that then where is the incentive to agree to give up a nuclear programme?
All in all, we've been joking for months that ww3 is starting whilst we're all sat in the pub, but it might not be much of a joke for much longer.
If Russia does annex the Crimea without a "legal" referendum of the people then it's an invasion. Then parties (ie France uk US) must live up to the obligations of their treaties and must give Russia a bit of a slap, and Putin isn't really going to like that very much is he?!