The only area in which Russians are a majority is Crimea. Ukrainians could give Russia serious problems merely conducting an insurgency, given the size of the country. If Ukraine decides to fight, Russia has just grabbed a hold of a tar baby. Given the capacity of Ukraine for resistance, this might be the war that finally cures Russia of its knee-jerk territorial acquisitiveness. Russia would neither be fighting a band of irregulars (Stepan Bandera) nor the nomadic raiders (Cossacks) of the past, but the organized military of a 50m strong population, with a Russian military that's a pale shadow of its Cold War peak.