What event in 1859 increased tension between the north and the south?

The event in 1859 that significantly increased tension between the North and the South was John Brown's raid on Harpers Ferry. Brown, an abolitionist, led a group in an attempt to seize a federal armory to incite a slave uprising. The raid heightened Southern fears of Northern aggression and the potential for widespread slave revolts, while many in the North viewed Brown as a martyr for the abolitionist cause. This incident deepened the divide between the two regions, contributing to the growing sectional conflict that would lead to the Civil War.