Russia and the U.S. #2: But why?

Here's the question:   Why is it our place to do something about Russia invading Ukraine?  Why isn't it at least the European Union's place -- if anyone's, other than Russia and Ukraine? 
This whole unrest in Ukraine flared up when their President Yanukovich cancelled the plans to have stronger ties with the EU and turned instead to Russia.  That's what led to the demonstrations and the ultimate overthrow of Yanukovich -- which led him to ask Russia to intervene in Ukraine.

So, yes, we have interests in having more countries operate as democracies;  and, yes, we want to stand up for equal rights and political freedom.   But Ukraine is far more important to Europe than it is to us.    Why shouldn't they be the ones taking the lead in the world's response?

Ralph