All you have to do is make the "yes" a "no" and the "no" a "yes" and your answer would be perfect.
Suppose you asked the average pro-war Northerner this: are you fighting to free the slaves? (Remember the title of the article.)
That Northerner would have honestly replied: Yes, and also to preserve the Union.
Now ask the average pro-war Southerner this: Are you fighting to keep slavery?
That Southerner would have honestly replied: No, I’m fighting for states rights.
Sure, you’ll find exceptions in both camps, but I stand by my original answer.