Some major events in Ukraine since I posted this a few days ago.

The pro-Russian president Yanukovych resigned, and fled to the majority-Russian eastern half of the country.

And Russia has mobilized troops on the eastern border of Ukraine, but says it's just "war games".

The U.S. (press secretary Jay Carney, and Defense Secretary Chuck Hegel) have warned Russia to tread lightly. I would worry about that more if the Obama administration had demonstrated the slightest spine in Libya, Syria or Egypt, or before selling out U.S. allies like Poland, Czech Republic, and Israel.
But as it is, I'm sure the Russians are laughing at the warning, and could take all of Ukraine before Obama's administration ever even presented a harshly worded letter.

And make no mistake, Russia --specifically Putin-- would like to reclaim major chunks of the former soviet union, especially Ukraine, Belorussia, Kazakhstan, Latvia, Estonia and Lithuania.

And while they might be crazy enough to seize them in the short term, Russia is a crumbling empire with a declining population, surrounded by southern neighbors with exploding populations, who will in coming decades increasingly migrate north.
They would be wise not to invade Ukraine or any other territory. They will have enough difficulty keeping the territory they currently have.