The main cause of the American Civil War was the issue of slavery, particularly its expansion into newly acquired territories and states. As the United States expanded westward, tensions grew between slave-holding Southern states and Northern states that opposed the extension of slavery. Economic differences, states' rights, and political power struggles fueled the conflict. The election of Abraham Lincoln in 1860, viewed as a threat to the institution of slavery, prompted several Southern states to secede from the Union, ultimately leading to the outbreak of the Civil War in 1861.