I will try to sum up what is going on. After Ukrainians made government turnover of president Yanukovych ( he and his sons have stolen over 40 billion $ during last couple years), the new government made a terrible mistake and cancelled pretty conditional and stupid law, which allowed russian language to be used in official documents ( Ukraine is mostly bilingual, and everyone understands both russian and ukrainian). Canceling this law doesn't mean that russian language is banned. It is just CANT be used in official documents. After canceling this law, parliamentary representatives told that there will be a new law which will respect not only two languages, but will spread on others which are used on Ukrainian territory.

Putin's propaganda presented this as infringement of ethnic Russians and that new government are fascists which made illegal turnover.

Three main nationalities in Crimea are ethnic russians (1.5m), ukrainians (500.00) and tatars (who actually lived in Crimea, before Stalin repressed them in 1944-1950). Also Russia pays Ukrainian government for keeping part of their fleet in Crimea.

So, these forces "invaded" Crimea, but the had no russian markings and it was again presented, as these are volunteers who help to protect Crimea from west-ukrainian fascists (yeah, right, volunteers with guns, helicopters and armored cars). The Crimea parlament was capruted and now is ruled by pro-russian marginals who declare Crimea autonomy and that is should be a part of Russia. The officially request help from Putin to protect themselves. And yesterday Russian Council of federations allowed Putin to invade Crimea. But post-faktum it already happened.