Slavery didn't become the war's focus for Northerners until later, after much blood had already been shed.
If you'd asked a secessionist politician in 1861 why he wanted secession, you might get an answer about "our Southern way of life" or "Southern rights," but scratch the veneer and you'd know what was underneath.
If you asked an average Southern soldier, he might say "state's rights" or "our way of life" or "the Yankees attacked us," or even "slavery."
I'll give the fighting man the benefit of the doubt, but those who started the secessionist movement and the war were pretty clear about what they feared and what they wanted.
Check out the secession declarations of the various states.
“Slavery didn’t become the war’s focus for Northerners until later...”
The Abolitionist Movement, Bloody Kansas, John Brown, The Compromise of 1860, The Missouri Compromise, the Underground Railroad, the 3/5 Compromise, the first Republican Platform of “Free Soil and Abolition”
I tend to think people knew what the war’s focus was.
Remember, when people speak of state’s rights in this context, they’re speaking about the states right to keep and sell human beings, among other things. If we believe in Natural Law, then there are no state’s rights in the context of slavery, or the “Southern way of life.”