The fact that the citizens of Crimea themselves want to break away from Ukraine certainly complicates things.
There is a potential for Civil War.
These questions of whether areas should be able to break away from a country and join another country or create a new country are always particularly sticky. On the one hand, you think the democratic process should decide. After all that’s what happened in Ukrainians recently. The U.S. is a particularly difficult position regarding this, because you know what the union’s stance was about this during the U.S. own civil war.
Obviously what Russia did was illegal. But if the people of Crimea themselves do not object, isn’t it hard to get worked up and feel the US should be involved?
This article is a good one, and worth the read. It talks about the legality of what is happening.
I think what is “legal” in the cases of revolutions is pretty much ad hoc and just depends on your own political interests.