Russia and the Crimea
I don't have any academic expertise on Ukraine, but I did go there for vacation in May 2010. While there, I was struck by the amount of pro-Russian sentiment and even Soviet nostalgia in the Crimean Peninsula. As you can see in the above picture, Yalta still has its Lenin statue, looking over a seafront promenade almost directly at a McDonald's with outdoor seating featuring Coca-Cola umbrella tables. People in the Crimean often prefer to exchange pleasantries in Russian rather than in Ukrainian, and you can still buy Soviet kitsch in the gift shops of Sevastopol.
Crimea today is not a normal Ukrainian state or province, but an "autonomous republic" which, ethnically speaking, is only about 25% Ukrainian. In the early 1990's, it considered trying to become a state independent of Ukraine, and continues to host a movement favoring as much autonomy from Kyiv as possible. The Soviets favored the region as host to the strategically critical Black Sea Fleet, which is still there under the Russian flag. Its beaches have also been a favored vacation ground for both Soviet and Russian elites.
All of this is to say that the Crimean conflict is not just about one nation-state moving into another that opposes it, but more like Russia taking a favored side in a long-running internal dispute which has become violent. As much as I prefer the Ukrainian pro-democracy forces to Putin's autocratic Russian nationalism, it would be a mistake to assume that the Crimea forms an automatic part of an independent Ukraine, or that the Crimeans want that. I suspect, unfortunately, that there are many who are glad to see the arrival of the Russian troops.