Why should Obama do something.
Let's review for those playing along at home. The Ukraine had a fair and free election, and a candidate won. That candidate then proceeded to get a really pretty good trade deal with Russia instead of the European Union. The European Union instead instigated factions not in the government to topple the legitimately elected president.
At that point, Ethnic Russians in the Ukraine, seeing that democracy didn't do them any good, decided they would rather be part of Russia than the Ukraine. If we put it to a province by province vote, like was done in the Crimea, the map would change radically.
Why is this our problem?