So what WAS it about?
George Grant relays these words spoken by Ulysses S. Grant, the last Northern General of the Civil War, and a slave-owner until it was illegal:
"If the war had been about slavery,' he [Ulysses] 'would have fought for the South."
George Grant continues:
"Ironically, his greatest on-the-field adversary, Robert E. Lee had stated exactly the opposite sentiment."
In other words, Lee would have said yes when the North asked him to lead them, if Lee thought the war was about slavery.