DoodleDawg
In response to your post, I agree with MrB’s sentiment:
‘It wasnt a civil war as the southern states had no designs on controlling the central government. Their design was to be independent of it.’
That’s why I consider the war an act of aggression.
But the South started it. That would make them the aggressors.
Endlessly repeated, that claim is essential to the pro-Confederate Big Lie.
In fact, the Confederacy had every intention of military expansion well beyond the borders of the eleven states which more-or-less legitimately voted to secede.
Those Union states & territories included Maryland, Kentucky, Missouri, Oklahoma, Arizona and New Mexico.
Indeed, one purpose for Lee's invasion of Pennsylvania in June 1863, was to control major northern cities & land, for possible use in future negotiations.
Confederates also launched raids or guerrillas into Ohio, Indiana, Kansas, Colorado, California and even Vermont, not to mention West Virginia.
So clearly the Confederacy represented a major existential threat to the entire United States, and that made its unconditional surrender unconditionally necessary.
Of course, just as you say, it was a "war of aggression" -- Confederate aggression launched against the United States of America.