FWIW, War Between the States always struck me as the most neutral term.
Civil War implies war within a nation, whereas the whole point of the war was over whether it was now one or two nations. The term at least implies acceptance of the single-nation idea.
There are also variants of more extreme terminologies. War of Northern Aggression vs. The Great Rebellion.
WBTS is most accurate, IMO, because to a very large extent that’s exactly what it was, especially in the beginning. The Federal and Confederate governments just did not have the machinery in place to run a war of such size, with much of the organization defaulting back to states as a result.
If Union states had just refused to support the war wholeheartedly, it would have quickly petered out.
“War Between the States “
Same here. When using the term “State”, that doesn’t mean a State of the USA but a national State. A union is not a State. So, the USA and the CSA went to war, using their States to do it.
States weren't fighting each other. It was two governments that claimed to be federal bodies composed of states.
I wouldn't want somebody to think that it was all about an evil federal government or gang of bad states beating up on good states (or vice versa).
I wouldn't want people to forget that the CSA laid claim to being a nation or country or federation of its own.