There was a concerted effort immediately following the Civil War to paint the war as about something other than slavery, and it took hold quickly even in the North. However, here's the fact in written records.
On Dec. 20, 1860, South Carolina became the first state to decide to secede. Their stated reason:
"[A]n increasing hostility on the part of the non-slaveholding states to the institution of slavery has led to a disregard of their obligations.
... [T]hey have denounced as sinful the institution of slavery. ... They have encouraged and assisted thousands of our slaves to leave their homes ... A geographical line has been drawn across the Union, and all the states north of that line have united in the election of a man to the high office of president of the United States whose opinions and purposes are hostile to slavery. He is to be entrusted with the administration of the common government because he has declared that 'Government cannot endure permanently half slave, half free,' and that the public mind must rest in the belief that slavery is in the course of ultimate extinction. ... The slave-holding states will no longer have the power of self-government or self-protection ... We, therefore, the People of South Carolina ... have solemnly declared that the Union heretofore existing between this state and the other states of North America, is dissolved, and that the state of South Carolina has resumed her position among the nations of the world, as a separate and independent state ..."
The rest of the Southern states soon followed, each citing the same reason. Yes, and yes again, it was all about slavery and self-rule to maintain slavery.
TOM WARD, Salina