We've been over this before. Merriam-Webster defines rebellion as open, armed, and usually unsuccessful defiance of or resistance to an established government. The confederate defiance was open, it was armed, and it certainly was unsuccessful. Considering that their acts of unilateral secession were also illegal then 'rebellion' is an accurate term.
Which is what you want them to be. Subordinate, yielding, teachable, pliable, malleable, docile, and above all, compliant.
That seems to be your preference, as well as other Southron types around here. You want us to be meek and mild and accept your opinion as fact and not argue the truth with you. And it really, really pisses you off when people don't roll over like that.
The secession acts of the People of the Southern States, with the arguable exception of that of Arkansas whose legislature acted for that State rather than the People, were and still are "legal" -- in fact, they are above "legal", they are supraconstitutional, beyond the reach, and beyond the scope and review, of any government or court. They were absolute and absolutely sovereign acts of the People for which the People answer to nobody who is not the Lord high invisible God of Israel.
By the way, there is a word for someone who thinks the Government is sovereign, the highest entity in society. Look that up in your Merriam-Webster's.