The Temperature Rises in Europe's Latest Hot Spot

Russian troops are poised on the Eastern Ukrainian border as the US and Russia trade charges of manufacturing political crises in that troubled country. Pro-Russian demonstrators are holding government buildings in the Eastern Ukrainian city of Donetsk, and a crackdown by the interim government could be the pretext for an invasion. Vladimir Putin has been scornful of economic sanctions imposed by the US and Europe after Russia annexed Crimea. How has such tension developed over Ukraine's industrial heartland? If Russia resorts to military action, what could the US do then? How dangerous is the situation?