Well, politically Russia will never go to war with Ukraine.
Russia, when it was known as the USSR, owned the eastern suburbs of Ukraine. It gave it to the state, meaning to take it back when it became more developed. However, the Soviet Union broke up in the 90's, and the eastern part has been Ukraine's, until now. The eastern side is the side the raises the GDP of Ukraine. It's the economically rich side, it has most of the factories that Ukraine makes its money off. However, in order to do this, Ukraine had to borrow a massive loan from the EU. European Ukraine wants to go back to Russia right now, and Russia is welcoming it back. However, (pardon me for the expression) if it goes back, the stuff hits the fan for the rest of Ukraine. They'll be the poorest country in Europe. The Eastern side will still bear the dept, which Russia will have to pay back. Because the western side is so screwed if this goes ahead, the US and the rest of Europe is strongly against this idea. However, as I said before, the eastern side and Russia both want this to go ahead. They're at a bit of a stand-off at the moment. Russia has sent in troops, basically as insurance. Remember last year when Kim Jong-Un went crazy? This is something like that. Russia will eventually figure out what they lost from this - 3 billion euros worth of goodwill from the Sochi games, and its face to the rest of the world.