"It was THE issue that caused a sectional divide between northern and southern states."

I cannot completely agree with that assessment. While Slavery was a cause of the sectional divide which you correct point out, it was the failure of the United States Government to protect the rights of the southern citizens as granted under the Constitution, that generated the mistrust of the Federal Government among the southern people that caused them to look at secession and forming their own government for the protection of their own interests.

It was that "Mistrust" caused by the failure of the Federal Government to enforce and protect the constitutional rights of that portion of its citizens wished to own slaves. Or if not to protect those rights, to change those laws and rights by legal due process, that caused the war. Not The Slaves, or slavery itself.

Even the Northern politicians, even Lincoln when he called for 75,000 Volunteers in 1861, said it was about "Preserving the Union" (Anti secession, keeping the southern states in the Union against their will) not freeing the slaves that caused the northern soldiers to enlist and to invade the southern states.

Sure abolitionist had other motives, but they did not have the numbers or power to enlist such a large army to conduct a war against the southern states without subterfuge.