The 'North' and the 'South' were not different nations. I'd base my argument around defining a nation (no easy job).

Then, go onto who benefits from calling it "War of aggression"? it’s a highly emotive name. In what ways did events fit that description?

And Who benefits from calling it "The Civil War"? It was indeed a civil war by all fair definitions, but has that term been used to gain some partisan political goal?
Who/and why do some people object to the title "War of aggression"?
HP

Were there 2 nations in the U.S. Civil War? The North felt that the nation was indivisible. The South cecded from the Union, set up their own government and capitol. Obviously, the South though they were a separate nation, the North did not.

The Civil War started when the South took the aggressive act of attacking Fort Sumpter. This being the case, I could see why some people might call it a War of aggression. If you favor the South however, you might be tempted to say that it was a War of aggression because the North invaded the South, but that argument ignores Fort Sumpter.

A Civil War is defined as a war internal to one country. Most people call it a civil war because it did involve the United States only, but again, if you define the Confederate States of America as a separate nation, then it doesn't fit the strict definition of civil war.

People probably do or don't want to call it a war of aggression for their own political reasons, but we have been calling it the Civil War for a long time, and I would suspect that most people find it easier to leave it at that.