The primary cause of the Civil War in the United States was the contentious issue of slavery. Tensions between the Northern states, which were generally anti-slavery, and the Southern states, which relied on slave labor for their agricultural economy, escalated throughout the mid-19th century. The election of Abraham Lincoln in 1860, who opposed the expansion of slavery into new territories, was seen as a direct threat by Southern states, prompting several to secede from the Union. These conflicts culminated in the outbreak of war in 1861, fundamentally shaping the nation's future and leading to the eventual abolition of slavery.