The issue of slavery became more and more contentious between northern states and southern states in the middle part of the 19th century, especially as the United States expanded westward and began to take in new territories and states. Slavery was important to the southern economy because of the large labor force required to pick and process cotton - the southern cash crop. The southern states scored a victory with the passing of the Kansas-Nebraska Act of 1854. Introduced by Stephen A. Douglas, the bill called for the residents of the particular territory to choose for itself on the issue of slavery (otherwise known as popular sovereignty). Northern politicians such as Abraham Lincoln and abolitionists (those opposed to slavery) fought vigorously against the bill. With the election of President Lincoln in 1860, southern officials began to fear that Lincoln would repeal the bill and that the northern majority would threaten their way of life – and their economic interests. Southern states began to fear that Lincoln would emancipate slaves. The 1859 raid on the federal arsenal by John Brown, a radical abolitionist, perpetuated fears and led many to believe in a northern conspiracy. Shortly after Lincoln's election, South Carolina officially seceded from the Union. Ten states would follow soon after and form the Confederate States of America.

Although the North was divided on their views of slavery, political officials in power such as Abraham Lincoln fought against the Kansas- Nebraska Act. Although many viewed slavery as immoral, which was one reason for its proposed abolishment, officials in the north were also worried that the extension of slavery into western states would give the southern states disproportional influence in such areas. When Abraham Lincoln was elected president, eleven states promptly seceded from the Union. Lincoln deemed this unacceptable and declared war for the purposes of preserving the Union.