Skydancer: “In my reading of history slavery didn’t become a factor or was made a selling point of the war until much later.”
But protecting slavery was the major factor driving Deep South declarations of secession, and major changes in their Confederate constitution.
Of course, slavery did not start Civil War, neither did secession or tariffs or anything else sometimes listed by pro-Confederates.
What started Civil War was the Confederate military assault on Union troops in Union Fort Sumter.
Ft. Sumter: Yes, that was a precursor to the WoS as well as the shelling a few months before of a Union supply ship by VMA cadets.