Russia can no more afford to let the Ukraine go than Canada can let Manitoba and Saskatchewan merge with the United States, or the US let Alaska go to Russia, or Texas to Mexico. The Ukraine is their western and southern frontier.
The United States has been insanely irresponsible in pushing the elected government of Ukraine to fall into the hands of the current revolutionaries, who are not the US-funded liberals whom the US thought they were supporting. Russia has been fighting to capture Crimea from the Tartars and the Ottoman Turks for several centuries; they will not let it go to some Western Ukrainian Catholics who no more represent the Russians of the Crimea than our Prime Minister Harper represents Quebec nationalists.
Two commentaries to which you should pay attention:
A former US Secretary of Commerce under Reagan, Paul Craig Roberts, gives a a very bleak interpretation of how much the Ukrainian revolution has been fomented by the US and how it has now fallen out of US control; and
Spengler on why the whole thing does not matter that much: “hopeless but not serious”.
Paul Craig Roberts would argue that it is both very serious and utterly hopeless, that Russia has absolutely vital interests at stake, and that our media are systematically misrepresenting how Russian actions are reasonable, and fairly mild, having regard for the forces and issues at stake. You should hear Roberts’ interview. It is shocking, but highly plausible.