I was a little child when Adolf Hitler invaded Austria. It was only from reading later that I learned he justified his conquest by saying it was really one country anyway, the same language and so on. Vladimir Putin could use the same arguments, I suppose. The headline in the morning paper has him saying Crimea is part of Russia.
The easiest time to stop Hitler, probably, would have been early, but nobody wanted to. Leaving it to later cost millions of lives.
So should the West go and hit Putin hard now? I can't imagine it--drop atomic bombs on a country that could drop them back? I can't imagine a Congress that would put its job on the line to do that.
Assassination--as in Osama bin Laden--might work but it's doubtful—he's too well protected.
If Putin really sees himself as the new Stalin, really wants to reconstitute the vast old empire, the rest of us face some very hard choices. We should start thinking about them soon.