The main cause of the outbreak of the American Civil War was the conflict over slavery. Tensions escalated between the Northern states, which were largely anti-slavery and industrialized, and the Southern states that relied on slave labor for their agricultural economy. The election of Abraham Lincoln in 1860, who opposed the spread of slavery into new territories, further exacerbated these tensions, leading to the secession of several Southern states. This fundamental ideological divide over the institution of slavery and states' rights precipitated the war, ultimately reshaping the nation and leading to the abolition of slavery.