Of course. It wasn't really a "civil" war since you didn't have two sides fighting for control of the country. The south just wanted to secede and form their own country.
It's still a civil war. There are two main types of civil wars it seems to me - ones in which one part of the country is fighting to rule the country/overthrow the government, and the other when part of the country wants to secede and is not allowed to.
Sometimes the rebels are more justified - and the south had some legitimate grievances - and sometimes they're less justified.
And for that matter there is no clear global standard for when a group should be allowed to secede. These issues are usually resolved by force more than principle.
Is every group - even individual - supposed to be allowed to 'secede' if they want, taking some of the land of a nation (usually, they can 'secede' by leaving).