The primary cause of the U.S. Civil War was the contentious issue of slavery. The differing economic interests and social structures of the Northern and Southern states created tensions that ultimately led to armed conflict. The South's reliance on slavery for its agricultural economy clashed with the North's increasing industrialization and growing abolitionist sentiment. As debates over state rights and the expansion of slavery into new territories intensified, the election of Abraham Lincoln in 1860—who opposed the spread of slavery—triggered the secession of Southern states, igniting the Civil War.