They did not attack the North to keep slavery. They stole federal property to keep the feds from doing fed things to them—for instance taxing—after secession. The theoretical war sparked by that incident, like I said, would have been different from the one we remember. The real war started because the feds decided to treat secession as an insurrection, and it was fought to force Southern states back into the union.
It was most emphatically NOT an invasion of North by the South to preserve slavery.
The South still wanted to keep its institution of slavery. The South can name its war with the US anything they want but The War Of Northern Aggression. The South did attack first. The feds replied in kind and fought to preserve the Union.