Russia’s conquest stands out for its bloodlessness. So far, Russian troops appear to have killed no one, or one person. This must be the most bloodless successful conquest in recent memory (assuming it does not escalate, which is unlikely with the withdrawal of Ukrainian forces) – and certainly the most bloodless per square mile. Even the massively overwhelmed Portuguese at Goa put up more of a fight, as did the Royal Marines in the Falklands, though there position was entirely hopeless. Heck, less blood was spilled over Crimea than over South Ossestia, Transdniester, or the Lachin Corridor, and no one knows what does are.
Though it is probably not fashionable to say so, this must in part be credited to the discipline of Russian forces. The lack of Ukrainian resistance will be much studied in the future. Obvious factors include the superiority of Russian forces, a collegial and national closeness with the other side (though that has not stopped bloody fighting in many a civil war), and the lack of of authority and legitimacy of the new, unelected Maidan regime.
Of course, there are many possible explanations for the lack of Ukrainian resistance. The most depressing would be if they had counted on some kind of greater Western support, and folded when it became clear no strong defense of the peninsula would come from third countries. Of course, nothing would have gotten NATO involved, but if one wants to try to encourage foreign intervention, as my colleague Jide Nzelibe has shown, it helps to die a lot first.