Which event significantly escalated tensions between the North and South prior to the Civil War?

The passage of the Kansas-Nebraska Act is a pivotal event that significantly escalated tensions between the North and South before the Civil War. This legislation, enacted in 1854, allowed the residents of the Kansas and Nebraska territories to decide for themselves whether to allow slavery within their borders through the principle of popular sovereignty. This effectively repealed the Missouri Compromise of 1820, which had maintained a balance between free and slave states, and it led to violent confrontations known as "Bleeding Kansas." Both pro-slavery and anti-slavery factions rushed to settle in these territories to influence the outcome, resulting in brutal conflicts and loss of life. This direct violence and the political upheaval that followed intensified the already existing sectional divisions and animosities between the North and South, contributing to the wider schism that would lead to the Civil War.