The primary issue leading to the American Civil War was slavery and its expansion into the newly acquired territories and states. Disagreements between the Northern and Southern states about whether to permit slavery in western lands intensified tensions. The Southern states relied on agriculture and slave labor, while the Northern states were increasingly opposed to slavery, advocating for its abolition. These sectional disputes culminated in the election of Abraham Lincoln in 1860, which many Southern states saw as a threat to their societal structure, leading to secession and ultimately the outbreak of the Civil War in 1861.