Sectionalism, states rights and slavery led to which event in American history?

The answer to this question is "secession and the Civil War." Southern secession was the result of a longstanding struggle over the issue of slavery, and especially the issue of slavery's expansion into the western territories. Long simmering, this issue first became truly divisive with the Missouri crisis over the admission of that state as a slave state in 1820. Resolved, like other crises involving slavery, by a political compromise, the crisis in many ways set the stage for future showdowns over the issue. The addition of the modern American Southwest through the Mexican War led to a new crisis, this time over the issue of California's admission as a free state (i.e., without slavery). The compromise that resolved this crisis included a very unpopular fugitive slave act. This act added momentum to the already rapidly-growing abolitionist movement in the North. Abolitionists had come to believe that slavery was irreconcilable with a free republic and especially with their religious principles.

In the aftermath of the Kansas-Nebraska Act, which opened the door to violent conflict in Kansas over slavery's expansion, the Republican Party was formed. Its main issue was the expansion of slavery, and when its candidate for President, Abraham Lincoln, won the national election in 1860, the state of South Carolina left the Union, followed over the next six months by 10 other slaveholding states. As we have seen, this was the culmination of a series of sectional disputes in which the South, a slave society, thought that its sectional interests were threatened by the North, where slavery was illegal. By 1860, when South Carolina left the Union, its politicians claimed that they had the right, as a state, to secede. What followed was a bloody four-year civil war in which the issues of slavery (but not really states rights) was decided once and for all.