Well, let's be precise, when you say "Civil War" are you referring to the war itself or to southern secession? The two are linked, obviously, but they each have different causes.
The southern states tried to leave the union to be able to continue to own human beings as slaves.
The southern states went to war to be able to leave the union to continue to own human beings as slaves.
The southern states were traitors to our nation at the time.
Anyone who complains about other nation flags being flown or football players taking a knee but also flies a confederate flag is nothing but a hypocrite. They are flying a flag of traitors to the United States of America. They killed hundreds of thousands of Americans just for the reason that they wanted to own human beings as slaves.
There is no other reason for the actions of the southern states and the civil war.
On top of that they lost and some conservative people today are still fighting that war.