The origins of the American Civil War lay in the complex issues of slavery, competing understandings of federalism, party politics, expansionism, sectionalism, tariffs, economics and modernization in the Antebellum Period.
The United States was a nation divided into two distinct regions separated by the Mason-Dixon line. New England, the Northeast and the Midwest had a rapidly growing economy based on family farms, industry, mining, commerce and transportation, with a large and rapidly growing urban population and no slavery outside the border states. Its growth was fed by a high birth rate and large numbers of European immigrants, especially Irish, British, German, Polish and Scandinavian.
The South was dominated by a settled plantation system based on slavery, with rapid growth taking place in the Southwest such as Texas based on high birth rates. There were few cities or towns, and little manufacturing except in border areas. Slave owners controlled politics and economics. Two-thirds of the Southern whites owned no slaves and usually were engaged in subsistence agriculture, but support for slavery came from all segments of southern society.
Overall, the Northern population was growing much more quickly than the Southern population, which made it increasingly difficult for the South to continue to control the national government. Southerners were worried about the relative political decline of their region because the North was growing much faster in terms of population and industrial output.
In the interest of maintaining unity, politicians had mostly moderated opposition to slavery, resulting in numerous compromises such as the Missouri Compromise of 1820. After the Mexican-American War, the issue of slavery in the new territories led to the Compromise of 1850. While the compromise averted an immediate political crisis, it did not permanently resolve the issue of the Slave power (the power of slaveholders to control the national government).
Amid the emergence of increasingly virulent and hostile sectional ideologies in national politics, the collapse of the old Second Party System in the 1850s hampered efforts of the politicians to reach yet one more compromise. The compromise that was reached (the Kansas-Nebraska Act) outraged too many northerners. In the 1850s, with the rise of the Republican Party, the first major party with no appeal in the South, the industrializing North and agrarian Midwest became committed to the economic ethos of free-labor industrial capitalism.
Arguments that slavery was undesirable for the nation had long existed. After 1840 abolitionists denounced slavery as more than a social evil — it was a moral wrong. Many Northerners, especially leaders of the new Republican Party, considered slavery a great national evil and believed that a small number of Southern owners of large plantations controlled the national government with the goal of spreading that evil.
In 1860, the election of Abraham Lincoln, who won the national election without receiving a single electoral vote from any of the Southern states, finally convinced Southerners that they were at the mercy of Northern politicians and that the time had come for self-government, which triggered their secession from the union.