Partition not inevitable in Ukraine
Many believe that an east-west partition is inevitable in Ukraine, but it probably won't be that simple. 78% of the country's 44 million people are ethnic Ukrainians, including majorities in the city of Kiev and all 24 oblasts (provinces). Ethnic Russians make up only 18% of Ukraine's total population, though they are a majority in Crimea, an autonomous republic, and in Sevastopol, a city not officially part of Crimea where Russia's Black Sea Fleet is based.
In Crimea, 42% of the population is not ethnic Russian, so if the occupation there turns into a guerrilla war, Russian troops will have plenty to worry about. And if Putin tries to extend his occupation to the provinces along the eastern border, ethnic Russians still make up no more than 40% of the population in any of them.
It should be noted that about a third of ethnic Ukrainians speak Russian as their first language, but that doesn't make them Russians any more than French-speaking Swiss are French or English-speaking Scots are English. Ethnic identity is closely linked to language, but it's not the same thing at all. Just ask the Anglophone Canadians, who would rather you not confuse them with Americans.
Something else worth considering is that Ukraine has the second largest army in Europe after Russia, and they don't have multiple divisions tied up in the Caucuses like Russia does. If it comes to blows between the two countries, don't discount the Ukrainians too much. Now that Ed Snowden isn't in a position to publish all of our military intelligence on the front page of the Guardian, a lot of it will make its way to the Ukrainian command staff. They will know exactly what the Russians are up to at all times. Also, remember that there are about a million ethnic Ukrainians in the US and another million in Canada. Their political influence in North America makes it unlikely that they'll be sold out to the Russians.