The outbreak of the American Civil War in 1861 was primarily caused by rising tensions over the issue of slavery, particularly its expansion into newly acquired territories and states. The United States was divided between the Northern states, which were largely anti-slavery, and the Southern states, which supported the institution of slavery for economic and social reasons. Key events such as the passage of the Kansas-Nebraska Act, the Dred Scott decision, and the election of Abraham Lincoln intensified these divisions. Ultimately, the firing on Fort Sumter by Confederate forces marked the official start of hostilities.