Although reports are sketchy, at this point there only seem to be Russian troops in Crimea, not in Ukraine proper.
But this begs the question, What, exactly, is Ukraine?
Ukraine, formerly the Ukraine (the Borderland), was really just that — a land on the border of Russia. It was not a distinct country (though the people were occasionally promised autonomy that never materialized) until the dissolution of the Soviet Union. The Crimean peninsula was only transferred from the control of the Russian Soviet Federative Socialist Republic to the Ukrainian Soviet Socialist Republic in 1954 by the government of the Union of Soviet Socialist Republics. This was an administrative move only. The people of Crimea remained largely ethnic Russians. Even now, Crimea is an “autonomous republic” (the only one) inside Ukraine. Sevastopol (which is on the peninsula, but is not part of the Autonomous Republic of Crimea) is home to both the Russian and Ukrainian Black Sea fleets. Russia has a rental agreement with Ukraine for that use.
Meanwhile, much of eastern Ukraine is ethnic Russian. We hear, quite reasonably, that the people of Ukraine should be able to choose their own path. But must they all go together? Ukraine’s borders are what the Soviet Union made them for its own convenience. If the people of Crimea decide they want to be under the auspices of Russia, should they be allowed to do so? Should they be allowed to be wholly independent — a new country — if they so desire? What of eastern Ukraine? Should Ukraine allow them to secede — and perhaps join Russia?
What if Ukraine says no? Who would fault the Russians for helping ethnic Russians in eastern Ukraine from joining Russia?
There are no easy answers here. I hope someone with more knowledge of the region will comment.