The main cause of the American Civil War in the 19th century was primarily the issue of slavery. The economic, social, and political differences between the Northern states, which favored a free labor system, and the Southern states, which relied on enslaved labor for their agrarian economy, led to increasing tensions. The election of Abraham Lincoln in 1860, who opposed the expansion of slavery, further escalated these tensions. Southern states felt threatened and began to secede from the Union, leading to conflict that would ultimately erupt into war in 1861. This fundamental clash over slavery and states' rights fundamentally shaped American history.