As a history nerd, I’ve often wondered how any war could be considered civil. I’ve recently found a logical explanation, and it goes all the way back to the Roman Empire.
Recall how most English words have more than one meaning. So it is with civil. The first definitions we usually think of are “quiet and peaceable in behavior,” “civilized,” and “mannerly.”
But there is an earlier definition — “of or relating to citizens.” This meaning originates in its Latin predecessor, civilis (from civis, meaning “citizen”). So in this context civil war becomes “a war between opposing groups of citizens of the same country.”
So you can have a civil war without the manners.
For a more detailed explanation, see “What’s So ‘Civil’ About ‘Civil War’?” ( https://www.merriam-webster.com/words-at-play/civil-meaning-why-is-it-called-civil-war? )