The Russian parliament voted unanimously to authorize military action in the Crimean Peninsula. Most of the peninsula is part of the Ukraine, but it is the most pro-Russian district. Only 28% of the population are ethnic Ukrainians. The majority are Russians, and most of the rest are Tartars. A small corner of the peninsula is leased to the Russian navy.
Pro-Russian militias have been blocking the bridges to keep the pro-Ukrainian demonstrators and the Ukrainian military from entering the district. Russia has mobilized naval forces and has a small amount of troops at the airports in Crimea.
Barack Obama and John Kerry seem willing to start WWIII over the Crimea, which the USA has no stake in. Crimea is just one of numerous territories left over by the Soviet Union that is in a state of ethnic/national limbo. Others include Transnistria (breakaway corner of Moldova, Abkhazia (breakaway chunk of Georgia), South Ossetia (besieged pro-Russian chunk of Georgia), Chechnya (Muslim district in Russia fighting for an Islamic state), and more. Estonia and Lithuania also have large Russian districts created by the Soviet Union to forcibly integrate the Baltics. These are also a source of major conflict today.
Of all the problems left over by the Soviet Union, some would say Crimea is an easy fix. A Russian plan to annex Crimea and grant easy Russian citizenship to ethnic Russians in the Ukraine could ease the conflict in the Ukraine.