Many Southerners opposed the rebel governments illegal secession, too. But the 'independent nation' existed only in the minds of those supporting the Southern rebellion.
A more fair and reasonable term would be that favored by my old Political Science professor, "The War of Federal Aggression".
The most accurate term would be the one used originally, "War of Southern Rebellion".
That's absurd. Secession was declared. It did happen. This is historical fact.
To deny this, is tantamount to claiming that in 1776 the independent United States of America was naught but a legal fiction existing solely in the minds of George Washington, Patrick Henry, and a few other malcontents.
It's simply absurd.
The most accurate term would be the one used originally, "War of Southern Rebellion".
(Shrugs). As you like it. The British termed the American Revolution "The Presbyterian Rebellion".
None of which changes the fact that any Individual may, at any time, freely Secede from his Government for any reason whatsoever -- or even for no reason at all (I have the Right to renounce my United States citizenship at any time, if I so desire).
What is true of any Individual is therefore, necessarily, logically true of any Group of Individuals (and therefore of any of the several States).
Ergo, unless you believe that the Federal Government, in response to an Individual's renunciation of his United States Citizenship, should immediately bring military force to bear against that Individual in order to compel him to remain subject to the US Federal Government, there can be logical argument against the absolute and unqualified Right of Secession for any reason whatsoever -- or no reason at all.