What was a major reason for the tension leading up to the Civil War?

A major reason for the tension leading up to the Civil War was slavery. The institution of slavery was deeply rooted in American society, particularly in the Southern states, and became a polarizing issue as the nation expanded. Debates over whether new territories and states admitted to the Union would permit slavery heightened sectional tensions between the North and the South. The South's economy relied heavily on slave labor for its agricultural output, while many in the North opposed the extension of slavery based on moral grounds and a belief in free labor.

The conflict over slavery was not just an economic issue but also involved deep-seated cultural and social differences. The ramifications of the Kansas-Nebraska Act, the Dred Scott decision, and the violent confrontations known as "Bleeding Kansas" further exacerbated divisions. Ultimately, slavery was not only a cause of conflict but also intertwined with other factors like industrialization and territorial expansion, making it the cornerstone of the tensions that led to the outbreak of the Civil War.