Well, the situation in the Crimea is a complex one. Allowing Putin to succeed in taking the Crimea (for whatever the current excuse is...it keeps changing) is just a "foot in the door".
The Ukraine is actually the birthplace of Russia. Most of the greatest battles fought in Russia's history happened in that region. The Russian Orthodox church has it's roots in the region and so on and so on.
There's already Russian troops on Ukraine soil, not just the Crimea.
With Putin's sentiments that the breakup of the Soviet Union being one ofnthe greatest tragedies in history, he is going to test the waters. If he is successful in nibbling away at the Ukraine's soverign territory like he is, then what's to stop him from going after any other region that has "Russian speaking" populations that need his "help", as was the case with the Crimea. Bear in mind that the demonstrations and unrest was in Kiev, not the Crimea. Putin said he was getting involved because of the demonstrations and unrest, but that was in Kiev and the mainland...it had nothing to do with the Crimea, and yet there they are...
So how about the other Russian speaking populations, like in Belarus, Rumania, Lithuania and Poland? Will they be next?
Looks a little like the late 1930's and "Lebensraum", doesn't it?