Has Ukraine happened before?

What would happen if we got involved militarily in the Ukraine? I think it's safe to say the same thing as would happen if the Russians got involved militarily in a country on our border. And we don't need to imagine that. It was called the Cuban missile crisis, and it is the closest we have ever come to nuclear war. 

Wouldn't it be insane to expect the Russians to act differently than we would in the same situation?