The primary factor leading to the American Civil War was the contentious issue of slavery. As the United States expanded westward, debates arose over whether new states would be slave or free, intensifying regional tensions between the North and South. The North, increasingly industrialized and leaning toward abolition, opposed the extension of slavery, while the agrarian South defended it as essential for their economy and way of life. Key events, such as the Missouri Compromise, the Dred Scott decision, and John Brown’s raid, exacerbated divisions. The election of Abraham Lincoln in 1860, viewed by the South as a threat to slavery, ultimately triggered secession and war.