“Most Southerners, even ones…
“Most Southerners, even ones who felt nationalistic toward the Confederacy, were not slaveowners.”
Of course they weren’t. In some parts of the Deep South, “most Southerners” were Black slaves.
But this misses the point. The question is about who the driving forces behind “the Southern cause” were, what they took the nature of their cause to be, and the reasons they gave for fighting for it.
And when you set out to answer that question, even a cursory glance at the public statements of men such as Jefferson Davis, Alexander “Cornerstone” Stephens, Robert E. Lee, and other members of the slaver aristocracy (who overwhelmingly dominated the secessionist conventions, the state governments, and the Confederate government)—not to mention at the Confederate constitution and other key sources—reveals that the prime motive of the people who were driving the process was the preservation of white supremacy and race slavery.
Certainly some whites who neither owned slaves nor were family members of anyone who did, still went out to fight for the Confederacy. But it’s essential to keep in perspective just how little sway they had in the reasoning or the decision-making that led into the hostilities that they later joined. And also that a lot of non-slaveholders weren’t interested in fighting for the Confederacy at all—which is why they were targeted by the “Twenty-Nigger” draft law, and why they led anti-Confederate uprisings across the large swaths of Virginia, Tennessee, etc. where the slaver population was very low.