Is it a psychological or intellectual barrier that won't allow some of your readers to get it?
The Civil War began with the secession of the Confederate states. The reason they seceded was that they believed that the entire basis of their economic life -- the slave system -- was threatened by the election of Abraham Lincoln. Confederate leaders stated this position clearly in Charleston in 1860.
Trying to quote Ulysses S. Grant to show that slavery wasn't the cause is bizarre, to say the least.
At Appomattox, Grant famously stated that he could not "rejoice at the downfall of a foe who had fought and suffered so much for a cause, though that cause was, I believe, one of the worst for which people ever fought."
In his memoirs, Grant wrote, "The cause of the great War of The Rebellion against the United States will have to be attributed to slavery. Slavery was an institution that required unusual guarantees for its security wherever it existed. Hence, the people of the South were dependent upon keeping control of the general government to secure the perpetuation of the protection of their favorite institution."
When Grant met Otto Bismarck in 1878, Bismarck said he had thought that the federal government fought "to save the Union." Grant responded: "Not only save the Union but destroy slavery. We felt it was a stain to the Union that men should be bought and sold like cattle."
Hilton Head Island