Answer: Taxation without Representation for the Colonies, Preservation of Slavery for the CSA.
Explanation: In the American Revolution, the main reason for independence was the idea that Parliament shouldn't have been allowed to levy taxes on the colonies because the colonies had no representatives IN Parliament. There were other factors, but this is the main one.
In the Civil War, however, the secession was to preserve slavery. Lincoln, a Republican, had just been elected, and the solidly Democratic southerners were under the notion that Lincoln was immediately going to ban slavery. (He wasn't, he wanted to stop its spread, and slowly kill it off). So they seceded in order to protect their "peculiar institution" and fired on Fort Sumter, starting the American Civil War. So really, it's not the "War of Northern Aggression", but the War of SOUTHERN Aggression.