A primary factor in the start of the American Civil War was the issue of slavery, particularly its expansion into new territories. Tensions had escalated throughout the 19th century as debates intensified over whether new states admitted to the Union would be free or slave-holding. The Missouri Compromise, the Kansas-Nebraska Act, and the Dred Scott decision highlighted these conflicts. Ultimately, the election of Abraham Lincoln in 1860, who opposed the expansion of slavery, prompted Southern states to secede from the Union, leading to the outbreak of war in April 1861. Slavery was at the heart of the economic, social, and political divides that fueled the conflict.