In the North it was “The Civil War”, to the Southern media and politicians it was “The War for Southern Independence”, to the those in the South who did the killing, the dying and the mourning, it was “The War Against Northern Aggression”.
C'mon, that sounds an awful lot more like something wiseacres and malcontents thought up much, much later.
Didn't it sound a little comical the first time you heard it?
Google finds it used exactly once during the war, by a Northern General as something he wanted to refute.
It looks like the phrase really got started in the 1950s and took off in recent decades. Source
Nowadays, somebody's going to accuse somebody like me of saying Confederate soldiers were stupid if I point out that "War of Northern Aggression" was pretty highfalootin' for use in the trenches and encampments, but ordinary folks, North or South, would have felt pretentious talking that way.
I can't say that nobody ever said, "This is a war of Northern agression," but that wasn't what soldiers commonly called the war.